The US Army has traditionally been composed of a number of branches, see this Wikipedia list: http://en.wikipedia.org/wiki/US_Army#Branch_Establishment

The major combat branches are Infantry, Artillery and Armor (originally Cavalry). Women cannot serve in these and other related front line combat branches, e.g. see http://en.wikipedia.org/wiki/Women_in_the_military.

Note for example that Sgt Leigh Ann Hester, the first woman to earn a Silver Star for "direct participation in combat", was a member of a MP company escorting a supply convoy. That woman who was captured and then rescued in the hotter part of the war was a member of a successfully ambushed supply convoy.

Also go back to that first Wikipedia page and look for where combat vs. support is used to describe. E.g. in the context of the new standardized combat brigade system (Heavy (mechanized infantry or tank), Stryker (wheeled death traps in the wrong terrain) and Infantry (light infantry or airborne)):

"In addition, there will be combat support and service support modular brigades. Combat support brigades include Aviation brigades, which will come in heavy and light varieties, Fires (artillery) brigades, and Battlefield Surveillance Brigades. Combat service support brigades include Sustainment brigades and come in several varieties and serve the standard support role in an army."