The Protoss (Khalani for 'First Born of the Gods') are a humanoid race from the planet Aiur. They are a warrior race that value honour, much like Elites or Turians. The Protoss have been important members of The League since they were accepted into it, following their War with the Zerg.
The Protoss were discovered on Aiur by the demented Xel'Naga, who decided to gift them with 'Purity of Form', whatever that is, making them counterparts to the Zerg. The Protoss are religious, believing in the Khala, which is very similar to The Force (All beings are linked together by a mysterious power). The Protoss' mastery of this technique allows them to communicate with any other being telepathically, but they have also evolved without mouths as a result. They also have very advanced technology, or at least thats what it seems, as they apparently built their monster flagship Protossima themselves.
The Protoss were involved in the three-way war in an rogue sector of the Milky Way, with the Zerg and the Renegade Humans. They basically took the Human side towards the end of the war, helping them attack Zerg worlds until eventually the Zerg sued for peace. This was caused by the Overmind's grand plan coming into effect and also becuase they were discovered by The League and negotiated with. Soon after, both Protoss and Zerg were accepted into the League and, despite destroying each other's Homeworlds, they seem to get on well. Protoss are common in the League' Army, fighting alongside Elites. They are often seen dueling with Elites pre-mission, a contest which both races enjoy, as a form of competition and a way to keep up morale.