Their main enemy are the Jedi, a force-sensitive order who wants to stop Sith and desires to keep the freedom of the people.
Sith players don't use blasters, but they can fight with the lightsabers, special "laser-sword" made of special crystals and held by Jedi too.
There are many lightsabers and if you are a sith you can use many styles of fighting.
In addition, Sith player's can unlock and use many powerful force-powers which will give to Sith huge advantages against their enemies, such as the Choke or Lightning.
Sith own the droid's army but they have many enemies: Jedi, clonetroopers, Bounty Hunters, non-sentient creatures and other characters.
On the Earth, you will find some Sith Outposts where you will be able to heal and rest, protected by other Sith.