The official missions of each branch give a pretty good example of what each branch's mission is:

• Army - "The Army’s mission is to fight and win our Nation’s wars by providing prompt, sustained land dominance across the full range of military operations and spectrum of conflict in support of combatant commanders."

Note: sustained land dominance. The Army's job is to take and hold land, and is thus geared for land combat on a large scale and for long duration missions.

• Navy - "The mission of the Navy is to maintain, train and equip combat-ready Naval forces capable of winning wars, deterring aggression and maintaining freedom of the seas."

The Navy is, not surprisingly, oriented towards naval combat in all aspects - from naval aviation to submarine warfare to special operations conducted on water.

• Air Force - "The mission of the United States Air Force is to fly, fight and win … in air, space and cyberspace."

The Air Force, also unsurprisingly, is focused on fighting in the air through gaining air superiority, which can only come with well trained pilots and well maintained aircraft. Also, the Air Force is focused on space and cyberspace, and has a lot of technical work in that area.

• Marine Corps - "The Marine Corps has been America's expeditionary force in readiness since 1775. We are forward deployed to respond swiftly and aggressively in times of crisis. We are soldiers of the sea, providing forces and detachments to naval ships and shore operations. "

The Marines are designed as naval infantry, and are thus oriented around combat on land from the sea. They work closely in conjunction with the Navy, who provides the ships capable of moving the Marine around the world. They're also designed as expeditionary forces, ready to deploy at a moment's notice (faster than the Army by design) with all the supplies and equipment needed to sustain themselves for a period of time.