Cisco's IOS uses the different
EXEC modes as a basic way to control user privileges. These user privilege controls can be delegated through an enable password, the local user database or AAA (RADIUS, TACACS+).
EXEC modes are user mode and privileged mode.
user EXEC mode> is limited to an array of show commands, basic reachability tests, such as ping and traceroute, as well as other ways of viewing configurations and status info of a Cisco device without the ability to make changes.
privileged EXEC mode# is for users that have been delegated admin privileges and need to make changes, view more show commands and debugs as well as using the ability to move further down the configuration hierarchy (such as global configuration mode, interface configuration mode, MQC, etc).
Additionally, both the user EXEC and privileged EXEC modes are subject to further controls known as privilege levels. By default the user EXEC mode has a privilege level of 1(includes all user-level commands) and the privileged EXEC mode has a privilege of 15(full privileges).
Here is a nice visual along with more information.