Avatar Manager is a plugin for WordPress that allows users or subscribers of your website to upload custom profile pictures or site generated avatars in your website.
Gravatar on the other hand is a built-in WordPress feature that allows the user to universally use their Gravatar registered profile especially when posting comments.
With the Avatar Manager plugin you can give your users or subscribers a choice of using their Gravatars or if they don’t want that, they can upload and manage their own avatars in your website.
Here is technical geeky code training on how to make an Avatar Manager plugin from scratch:
With the Avatar Manager plugin installed and active, users should be able to see the options under the Users>User Profile>Avatar section of the menu. They will be given the option to upload or use previously uploaded media files as their avatars.