In WordPress, each user account on the website has his/her own avatar, but to set this avatar, users must use a tool called Gravatar. This work sometimes costs time and effort. However, you can completely change it by creating a field that allows users to upload photos right on the website, then set that image as an avatar. Thus, user’s manipulation becomes somewhat simpler. In this case, the avatar that the user has set himself/herself (not by Gravatar) is temporarily called a custom avatar.
WordPress allows you to add more users to your website, but it’s from the backend only. If you want to allow visitors to register and manage their accounts from the frontend, you might create a custom sign up, sign in, and user profile page for them. You also might create some fields for them to fill in their information. If you are using Meta Box plugin, you may use it to do it easily.