The Army exists to kill people and break stuff. Sure, they may be called upon to perform other missions like peace keeping, peace enforcement, training, and disaster relief, but these secondary missions are temporary and contribute to the goal of protecting the ROD, her allies, and her interests world-wide.
The Army consists of both active duty and reserve units. Active duty brigades and divisions are made up of full time soldiers: on duty 24/7. Reserve units are civilians who drill with their military units one weekend every month, plus a 2-week training even over each summer. However, reserve units can be activated in time of war or national emergency.
The Army has organized most of its units into two corps: one active duty and the other reserve. I Corps, with the exception of the corps headquarters and Corps Artillery, is a fully reserve formation. II Corps is 100% active duty.