Crash Course Category Theory is an open source collection of notes and resources dedicated to the study of Category Theory! If you have never heard of Category Theory but want to learn more about it, this website is the perfect place to start.
A layperson's description of category theory can be summed up as follows:
At its core, category theory is the study of objects in a group (i.e. category) and the relationships (i.e. morphisms) those objects have with one another.
There are a variety of resources available on this site! Please visit the resources page for information on books, software, and more.