All Machine Learning algorithms explained

Written by Massa Medi| May 1, 2025

Feeling lost in the labyrinth of machine learning algorithms? Whether you’re a curious beginner or a data pro looking to refresh your knowledge, this guide will break down the most important machine learning (ML) algorithms — from linear regression to neural networks — in a way that’s intuitive, actionable, and jargon-busting. By the end, you'll be equipped to confidently choose the right algorithm for any problem, understand the core intuition behind each, and see how they relate in the vast universe of AI.

Meet Your Guide: Tim, Data Scientist & ML Instructor

Hi, I’m Tim. With over a decade as a data scientist and hands-on experience teaching these concepts to hundreds of bootcamp students, I’ve distilled everything you need to know about major machine learning algorithms into this comprehensive roadmap. If you’re overwhelmed by all the buzzwords, don’t worry — that ends now!

What is Machine Learning?

Let’s start with the big picture. According to Wikipedia, machine learning is a field of AI focused on developing algorithms that learn from data, generalize to new situations, and perform tasks without explicit programming.

Most recent leaps in AI — think self-driving cars, voice assistants, or mind-blowing image generation — are fueled by machine learning, especially neural networks. But before we get into that, we’ll break machine learning into its key fields.

Machine Learning: Supervised vs. Unsupervised Learning

At its core, ML comes in two main flavors:

1. Supervised Learning: Here, you have a dataset with known “correct answers” (labels). Think of this as showing a child what a cat is, what a dog is, and then asking them to identify a new animal based on what they’ve learned.
   • Example 1: Predicting house prices (e.g., based on square footage, location, year of construction, etc.).
   • Example 2: Classifying whether an object is a cat or a dog based on features like height, weight, ear size, and eye color.
2. Unsupervised Learning: No labels, no instructions — just raw data. You let the algorithm group things based on similarity, with no hints about what’s what. Picture dropping a stack of photos in front of a kid who’s never seen a cat or dog, then asking them to sort the images into groups however they like.
   • Example: Automatically sorting emails into unspecified categories (clusters), which you can later inspect and label.

Supervised Learning: Regression vs. Classification

The lion’s share of ML work happens in supervised learning, which has two major jobs:

• Regression: Predicting a continuous number (e.g., the price of a house).
• Classification: Predicting a discrete label (e.g., whether an email is “spam” or “not spam” — or classifying emails as “primary,” “social,” “promotions,” etc., as Gmail does).

Foundational Algorithms in Supervised Learning

Linear Regression: The OG of Prediction

Linear regression is the grandparent of machine learning algorithms — simple, powerful, and the building block of much fancier methods. It tries to fit a straight line through the data by minimizing the sum of squared distances between your measured points and the regression line (the so-called “least squares” approach).

Example: Imagine correlating a person’s shoe size with their height. The fitted line might say “for each increase in shoe size, height goes up by around 2 inches.” Add more features (like gender, age, or ethnicity) for a richer, multi-dimensional model — but the core idea remains: learning the relationships that help us predict an output.

Fun fact: Many sophisticated algorithms (even neural networks!) are just evolved versions of this basic idea.

Logistic Regression: Fast-Track to Classification

Logistic regression upgrades linear regression for classification problems, typically assigning binary labels (yes/no, spam/not spam, etc.).

Instead of a line, we fit a special curve called a sigmoid function, which maps inputs to probabilities between 0 and 1. For example, it might tell us “an adult who’s 180 cm tall has an 80% probability of being male” (made up statistic, but you get the idea). Logistic regression is a staple for predicting categories when relationships are straightforward.

K-Nearest Neighbors (KNN): Lazy, But Effective

KNN is a wonderfully intuitive algorithm that skips traditional modeling. Instead, for any new data point, we look at the k closest known data points (its “neighbors”) and let their values (class, average, etc.) decide our prediction.

Example:

• Classification: To predict gender, find the five people closest in height and weight, and go with the majority’s gender.
• Regression: Estimate a person’s weight by averaging the weights of the three most similar people.

The magic value “k” is a hyperparameter that you adjust for best performance. Pick too small a k, and you might “overfit” (your model memorizes data quirks rather than general rules). Pick too big, and you “underfit” (the model becomes too generic, missing important distinctions). Data pros use cross-validation to find just-right values for k.

Support Vector Machine (SVM): Drawing Sharp Boundaries

SVM algorithms create boundaries that partition your data into classes, seeking the cleanest possible split. Picture plotting animals by their weight and nose length — the SVM draws a line (or in higher dimensions, a “hyperplane”) that separates, say, cats from elephants, maximizing the space on either side to avoid misclassifying strays.

Support vectors — those data points at the margins of the split — are what SVMs actually “remember,” making them super memory-efficient.

The real power comes from kernel functions, allowing SVMs to draw non-linear boundaries by transforming the data into higher dimensions behind the scenes (the “kernel trick”). That’s how it can tackle tough, twisty problems with finesse.

Naive Bayes: Statistical Simplicity

Named for its (purposefully) naive assumption of independence between features, Naive Bayes classifiers are lightning-fast. They’re a classic for spam filters: you train them by counting word frequencies in spam and non-spam emails, then use those probabilities (thanks, Bayes’ Theorem!) to classify new emails. Despite the “naive” label, they perform surprisingly well on many text tasks where speed counts.

Decision Trees & Ensembles: Simple Rules, Big Power

Decision trees break decision-making down into a series of yes/no questions — for example, “Is the patient’s cholesterol above 200?” or “Does the email contain the word ‘lottery’?” The aim: create “leaves” (end branches) that are as “pure” as possible, containing mostly one kind of label.

Ensemble methods take many basic decision trees and combine them for stronger models:

• Bagging & Random Forests: Randomly train multiple trees on different data subsets and have them “vote” on the outcome. Randomization makes them robust and prevents overfitting.
• Boosting: Train a series of trees in sequence, each one learning from the mistakes of the last. This approach (adopted by algorithms like AdaBoost, Gradient Boosting, and XGBoost) often achieves even higher accuracy but requires more careful tuning to avoid overfitting and takes longer to train.

Neural Networks & Deep Learning: Learning Hierarchies

Neural networks are inspired by the brain's interconnected neurons. They extend the “feature engineering” ideas from SVMs and decision trees to a new level. Instead of hand-crafting features (like “is there a vertical line in this image?”), neural networks learn these abstractions automatically by stacking multiple processing “layers.”

How does this look in action?Imagine trying to classify pictures of handwritten numbers. A simple logistic regression would struggle, since everyone's “1” looks different. But a neural network can “discover” features like “verticalness” or “no circular shapes,” even when you don't spell them out. The input pixels feed into hidden layers which transform them into ever more abstract representations — perhaps recognizing lines and shapes, and finally associating them with digits from 0 to 9.

Add more hidden layers, and you've entered the realm of deep learning — where the network finds patterns humans might never notice. We rarely know exactly what each hidden layer learns, but the end result is powerful, flexible predictions.

Switching Gears: Unsupervised Learning

Sometimes, you don’t have labeled data and just need to find patterns or structure. That’s the territory of unsupervised learning.

Clustering: Finding Hidden Groups

Clustering and classification might sound similar, but they’re fundamentally different. Classification uses known labels; clustering makes discoveries in unlabeled data.

Clustering in action: Imagine plotting dots on a graph that naturally cluster together. The K-means algorithm tries to find k such clusters by:

1. Randomly picking k “centers” in the data.
2. Assigning each point to the nearest center.
3. Recalculating the centers based on assigned points.
4. Repeating until centers stop moving.

Picking the right value of k is both an art and a science, depending heavily on your specific data and goals. Other clustering approaches — like hierarchical clustering or DBScan — can detect clusters of any shape without predetermining the number, but these are more advanced topics.

Dimensionality Reduction: Trimming the Fat

Real-world data can be massive and messy, with tons of features (think “columns” in a spreadsheet) — but not all of them carry unique information. Dimensionality reduction techniques, like Principal Component Analysis (PCA), find correlations, blend features, and keep only what matters.

Example: Predicting fish species by features like length, height, color, and number of teeth. If length and height are strongly correlated, PCA might collapse them into a single “shape” axis. Each principal component represents a direction in feature space where variance is highest, helping you simplify your dataset without losing much accuracy.

Pro tip: Dimensionality reduction is also great as a pre-processing step in supervised learning, making models faster and more robust by reducing noise.

Summary: Choosing the Right Algorithm

If you're still not sure which algorithm suits your problem, don’t fret — there’s an excellent cheat sheet by Scikit-Learn that maps out the decision process visually.

Machine learning can seem daunting, but remember every fancy model builds on the same foundational concepts. With these explanations, you’re ready to dive deeper — check out my roadmap on learning machine learning for step-by-step guidance.