The F/A-18B, F/A-18D and F/A-18F are two seaters, used for operational conversion training. They give up some fuel capacity and/or some bombload because of the second cockpit, but otherwise have the same capabilities as the single seaters. IIRC, the Marines use some of the two seaters for regular missions where, somewhat in the fashion of the Air Force F-15E, the extra set of eyes and the extra brain are needed. Otherwise the two seaters can just fly regular missions with the second seat empty.
F-15 and F-16 also have two seat varients, -B and -D models. The F-15E is a somewhat different bird, optimized for long range strike rather than air to air combat. (This in spite of the motto of the Air Force developement and fighter communities during it's design "Not a Pound for Air To Ground".)