In plain English
Machine learning is a subset of AI in which systems learn patterns from data and improve with experience, rather than being explicitly programmed for each task. Instead of a person writing the rules, you show the system many examples and it works out the rules itself. Most of what people call AI today is built on machine learning.
Why it matters
This shift, from writing rules to learning from data, is the core idea that makes modern AI possible and explains why data quality matters so much.
A worked example
Rather than coding every rule for spam, you show a system thousands of emails already marked spam or not, and it learns the patterns that distinguish them.
Common confusion
All machine learning is AI, but not all AI is machine learning. Some older AI systems follow hand-written rules with no learning involved.