Full Combo, more commonly abbreviated as FC, is the term that refers to beating a song while correctly playing every note without overstrumming. This is different from simply scoring 100% on a song, since one can "break" (reset) their Multiplier, by overstrumming, and still play 100% of the notes. Every single song in the entire series has been FC'd by at least one person, with the exception of Trogdor (NTSC). The NTSC version of Trogdor has been thought to be impossible due to the strumming speed of a part of the song exceeding the strum limit of the NTSC version of Guitar Hero II. However, player AbyssionQC was able to FC the solo to Trogdor on the NTSC version of Guitar Hero II on an emulator, therefore potentially proving that an NTSC FC is possible. You cannot FC songs if you suck at the game.
FBFC - also known as Full Band Full Combo, meaning that the maximum amount of people in a band (in a majority of cases, guitar, drums, bass, and vocals) FC a song without any duplicate instruments if the song does not include all four instruments.
OMBFC - One Man Band Full Combo, meaning that that each instrument has been FC'd by the same person.
FGFC - Full Game Full Combo, meaning that a player has FC'd every setlist song, not including DLC.
FSFC - Full Series Full Combo, meaning that a player has Full Game FC'd every game in the series.
On Guitar, an FSFC has been achieved by 5 people: supermarble96 was the first to obtain an FSFC, followed by kramral, Preston Helms, megapengproductions, and A Wise Moose. (GHAddict112 is missing just Pandora's Box from GHA)
On Bass, an FSFC has been achieved by 3 people: superspaz117, Preston Helms (again) and RobGHH.
On Vocals, an FSFC has been obtained by 3 people: Siden Entertainment, MrWhYYZ, and SneakCoot.
On Drums, an FSFC has been obtained by 3 people: smarockRB, dayzeed, and JoelD77. Interestingly, dayzeed has also FSFC'd the Rock Band series on drums, and is the first ever person to have obtained an FSFC on both the Guitar Hero series and the Rock Band series.