The leaves of a tree or plant are the parts that are flat, thin, and usually green. Many trees and plants lose their leaves in the winter and grow new leaves in the spring.

A leaf is one of the pieces of paper of which a book is made.

If you take a leaf from someone's book you behave in the same way as them because you want to be like that person or as successful as they are.

If you say that you are going to turn over a new leaf, you mean that you are going to start to behave in a better or more acceptable way.

If you say that someone is shaking like a leaf, you mean that their body is shaking a lot, for instance because they are very cold or frightened.