A binder is a kind of notebook meant to hold loose papers together. You might have a binder for each of the classes you're taking.
One type of binder has three metal rings that keep papers securely in place, and hard plastic covers to protect them from being bent or torn. You can also use the word binder when you're talking about a sticky or adhesive substance, or anything that binds two items together. The Old English root is bindere, "a person who binds," from bindan, "to tie up."