The game story is set to about twenty years after the events seen in popular TV series.
In essence, the Federation and the Klingon Empire are once again at war, and those are also the two main playable factions in Star Trek Online.
Once you begin playing STO however you only have access to one faction, the Federation.
After reaching level (rank) 5, you can choose to continue playing as a Federation officer or a Klingon warrior.
The two paths are nothing alike so be careful which one you choose.
The content you will experience with each of those factions is completely different, as Federation has access to a lot of PvE content (quests, exploration, etc.
), Klingons are all about PvP (player versus player) combat.
That being said, choosing a particular faction also has a great impact on leveling your character.
If you're not the kind of player who likes to poke into every battle you might want to play Federation, rather than the bloodthirsty Klingons who's main objective is to annihilate everyone.
PvP focused players will on the other hand enjoy the endless pursuit of other players.
The main issue here lies with players who prefer a different play style than the one set by Cryptic.
For example, Klingons are brave warriors who fear nothing and many players enjoy having such a background, however not all want to fight other players constantly.
When leveling as a Klingon you don't have many options and that's the biggest setback and will most likely mean a lack of players on this faction.
Regardless of which faction you choose to play, you will have to level up one way or the other.
I recommend checking out some leveling guides first as they are an incredibly useful asset for leveling any character no matter the faction or profession (science, tactical or engineering).
These leveling guides will undoubtedly help you gain ranks much faster than other players and in turn let you experience Star Trek Online's endgame a lot faster than your adversaries.