User Access Manager and Role Manager Plugins are used to assign access to the wordpress user. But have some different features and they are as under:
User Access Manager:
If you just want to control access to your pages & posts, the way to go is User Access Manager. This allows you to set up posts and pages that are only accessible by selected users. This is a great way to organize group projects or pages for individual clients, employees, etc.
If you want to control which roles have access to which admin features, then the way to go is Role Manager. This plugin allows you to see and modify all of the default roles and their associated capabilities. You can also create your own custom roles and capabilities, or assign new capabilities created by other plugins to the desired role(s).