[rattlsnak]

OK, lets say, *most* of the dual seaters are trainers.

A/B
The F/A-18A is the single-seat variant and the F/A-18B is the two seat variant. The space for the two seat cockpit is provided by a relocation of avionic equipment and a 6% reduction in internal fuel; two-seat Hornets are otherwise fully combat-capable. The B model is used primarily for training.

C/D
The F/A-18C is the single-seat variant and the F/A-18D is the two seat variant. The D model can be configured for training or as an all-weather strike craft. The "missionized" D's rear seat is configured for a Weapons and Sensors Officer to assist in operating the weapons systems. The D model is primarily operated by the U.S. Marine Corps in the night attack and FAC.

E/F Super Hornet

The single seat F/A-18E and two-seat F/A-18F Super Hornets carry over the name and design concept of the original F/A-18, but have been extensively redesigned. The aircraft is currently in production and will eventually equip 22 squadrons. The EA-18G Growler is an electronic warfare version of the two-seat F/A-18F, which entered production in 2007. The Growler will replace the Navy's EA-6B Prowler.


Other US variants
RF-18D
Proposed two-seat reconnaissance version for the US Marine Corps in the mid-1980s. It was to carry a radar reconnaissance pod. The system was canceled after it was unfunded in 1988. This capability was later realized on the F/A-18D

X-53, NASA's modified F/A-18.TF-18A
Two-seat training version of the F/A-18A fighter, later redesignated F/A-18B.
F-18 HARV
Single-seat High Alpha Research Vehicle for NASA.

Export variants

F-18L
This was a lighter land-based version of the F/A-18 Hornet. It was designed to be a single-seat air-superiority fighter and ground-attack aircraft.

F/A-18A: Single-seat fighter/attack version for the Royal Australian Air Force.
F/A-18B: Two-seat training version for the Royal Australian Air Force.

CF-18A : Single-seat fighter/attack version for the Canadian Forces. The Canadian Forces' official designation is CF-188A Hornet.
CF-18B : Two-seat training and combat version for the Canadian Forces. The Canadian Forces' official designation is CF-188B Hornet.

EF-18A: Single-seat fighter/attack version for the Spanish Air Force. The Spanish Air Force designation is C.15.
EF-18B: Two-seat training version for the Spanish Air Force. The Spanish Air Force designation is CE.15.

KAF-18C: Single-seat fighter/attack version for the Kuwait Air Force.
KAF-18D: Two-seat training version for the Kuwait Air Force.