A attack: attacks ground targets usually in close air support of ground troops

B bomber: attacks ground targets by dropping bombs

C cargo: Airlifts materiel and/or personnel

E electronic: denies electronic Spectrum to the enemy and protects US forces use of the Spectrum

F fighter: attacks other aircraft to win air Supremacy

Hx search and rescue: locates and retrieves isolated personnel

xH helicopter: rotary wing aircraft

K tanker: provides in-flight refueling for other aircraft

Mx multi-mission: performs a number of missions usually of a special operations nature

O observation: Observe and Report tactical information

P Maritime Patrol: braids for long durations in anti-submarine, anti-shipping and search rolls

Q Unmanned: operates without an onboard aircrew

R reconnaissance:obtains information about an enemy

S anti-submarine: finds, tracks, and attacks submarines from the air

T trainer: trains Pilots, Navigators, and other aircrew

U utility: forms several General purpose roles
( the U-2's designation misleading it is a reconnaissance aircraft)

Vx staff transport: Airlift of high-level military and government offcials

xV VTOL: fixed-wing aircraft that have vertical take-off and Landing capability

W weather: supplies timely and accurate Metro location information

X experimental: aircraft that include new technologies

Y prototype: the first model(s) of an aircraft which are used for testing purposes

Multi-letter designation, some aircraft have two letters in there designation, indicating that they perform multiple roles. the first letter represents the primary function. example

Ac-130 an attack aircraft that is based on the C-130 cargo aircraft
Kc-135 a tanker aircraft that also can perform a cargo mission
Fb-111 a fighter aircraft that also functioned as a bomber (retired)