In general, the last letter(s) announces the ship type, the first one(s) announce the owner. Shortly:
GT = Galactic Terran
PV = Planetary Vasudan
GV = Galactic Vasudan
GTVA = Galactic Terran-Vasudan Alliance (AFAIK only used as a prefix to GTVA Colossus)
S = Shivan (duh)
NT = Neo-Terran
F = Fighter
B = Bomber
C = Cruiser
Cv = Corvette
D = Destroyer
SD = Superdestroyer
J = Juggernaut
Tr = Transport
Is there any others?
Thus, GTD is Galactic Terran Destroyer and GTF would be Galactic Terran Fighter, but GTF and GTB are usually only used as type designations, not parts of ship names because fighters and bombers are using squad markings instead of individual names.