User groups make it simple to set up access to various portions of the system. Instead of setting individual access rights for every user on every screen (which you can do... if you want) set up user groups for the different levels of security and assign security levels to those groups. Then you can simply assign users to a user group and all of their access rights will be set accordingly.
For Instance, you might set up one security group called Sales, and another called Production. A third group, say Shipping, might have the lowest access rights of all.
See User Security