You can choose any species and background for you character, however, your backstory must be reasonable, bearing in mind you're a new recruit starting at the bottom of the rank scale. You are unlikely to have any spectacular military history.
Once a detail in your biography has been set, you can't change it back. So think carefully about it.
You are free to play your character any way you like, but try to keep it consistent!
You're only required to post 'in character' for RPGs, all other threads are 'out of character'.
You are free to use other players' characters in your posts, but you should try to remain true to their character. You shouldn't try to develop other players' backstories unless you have something planned with them.
It is worth noting that many player characters have their species listed as Tau'ri, meaning they're from Earth, a slightly different species to Humans from the rest of the Galaxy. Characters from Earth are no longer allowed, as this is a Star Wars game above all.
Non-Player Characters are characters in the game who are not played by real people, they are completely fictional. Rogue Squadron is populated by many NPCs, which are listed on the roster with red names rather than blue.
Anyone can use existing NPCs in their posts, or create their own. To create an NPC, simply refer to them by name in an official RPG. Once spotted by command, they will be added to the roster and you will be able to flesh them out by clicking the edit link on their profile.
To avoid flooding the unit with unnecessary NPCs, try to interact with players where possible, or use an established NPC.