In VRChat, you can use and create custom avatars!

Creating Avatars

There's a whole Avatars category on the sidebar to check out! Here's some of the more impactful and important pages:

Avatars 3.0 - Learn about our Avatars 3.0 SDK!
Creating Your First Avatar - The guide linked above, it's a very generalized guide for first steps!
Rig Requirements - Wondering how to rig up your avatar in a way that VRChat expects? Here's where to learn about it!
Avatar Performance Ranking System - Wondering where all your frames went, and why your avatar is ranked as Very Poor? Check out this page to learn why!
Avatar Optimization Tips - Now that you know why, check out this page to learn how to get all your frames back.