User accounts can be restricted using the permissions checkboxes so that they can only access certain areas in the back end. The back end is divided into four broad areas:
Configuration permissions - user can change config values, shipping, tax, countries and other setup details
Product permissions - user can view/edit categories, products and versions
Orders permissions - user can view/change customers, orders, affiliates and coupons
Support permissions - user can view/change support tickets and knowledgebase articles
This permissions structure serves two purposes. Firstly it allows you to prevent certain users from being able to make major changes to site settings, or products, for example, while allowing them to process orders. Secondly, it simplifies the back end somewhat by removing extraneous menus for users who do not need them.