Series
TOS-Star Trek: The Original Series. Before the movies and later series it was simply 'Star Trek' with no further desinator.
TAS-Star Trek: The Animiated Series
TNG-Star Trek: The Next Generation
DS9-Star Trek: Deep Space 9
VOY-Star Trek: Voyager
ENT-Star Trek: Enterprise
Movies
TMP-Star Trek: The Motion Picture
TWK-Star Trek II: The Wrath of Khan
TSFS-Star Trek III:The Search for Spock
TVH-Star Trek IV: The Voyage Home
TFF-Star Trek V: The Final Frontier
TUC-Star Trek VI: The Undiscovered Country
Generations-Star Trek VII: Generations
FC-Star Trek VIII: First Contact
Insurrection-Star Trek IX: Insurrection
Nemesis-Star Trek X: Nemesis
2150-2265
Pre TOS is before The Original Series. Theoretically this is the period of Star Trek: Enterprise (2151-2154), but since that series doesn't exist in my games, it isn't.
2266-2270
TOS Era is the time period from slightly before The Original Series to slightly after The Animated Series. This the baseline for Star Trek, and the last refuge of original fans.
2271-2286
This is the time from slightly before Star Trek: The Motion Picture to slightly after Star Trek IV: The Voyage Home. Many FASA gamers consider this to be the height of the franchise.
2287-2325
This is the period immediately following the first 4 movies. Most of the ships and technology will be upgraded versions of TMP era material.
2326-2363
This is the era where we begin to see the deployment of new technologies, and the ships begin to change in appearance becoming more like the TNG era vessels. This era leads into the Star Trek of TNG, DS9, and VOyager.
2364-2377
This is the time period from slightly before Star Trek: The Next Generation to slightly after Star Trek: Voyager. Mostly favoured by younger people who became Star Trek fans in the late 1980's. Some fans, and gamers consider these series to be different eras. I don't because they overlap chronologically both in the real world, and the fictional world, and are effectively similar. Technology doesn't change much, and the socio-political situation remains the same, except for changes actually made within the series.