Even when they’re not pure black boxes, machine-learning algorithms can sometimes become complex enough that even their creators don’t fully understand them.

From Apple to Google to Toyota, companies across the world are pouring resources into developing AI systems with machine learning. This comprehensive guide explains what machine learning really means.

As it currently stands, the vast majority of the AI advancements and applications you hear about refer to a category of algorithms known as machine learning (see "What is machine learning?").