From Valve Developer Community
Previously known as alien slaves, vortigaunts, or vorts, these are the Xen aliens first seen in Half-Life. They seem to have mastered electrical surges, and can charge themselves up to disperse energy, from great jolts of damaging current, to a steady flow that can recharge devices (like Gordon's suit).
Quotes from HL2 suggests that the vortigaunts were controlled by the Nihilanth for eons much like Breen controlled the human race: The Nihilanth was a Combine product, and the vorts mostly rejoiced when it died, but it was only "the lesser master" and they now work together with Freeman to kill "the greater master".
The vortigaunt seem to refer to "a deeper mystery" and "something secret steers us both", which seems to refer to the G-Man, so they seem to have been contacted by the G-man too, or at least someone from his origin. The vorts have made the stand to fight against the Combine or be extinct: They think their lives are worthless unless spent on freedom.
- This is the Freeman. The Combine's reckoning has come.
- We never dreamed to meet you in corporeal form.
- The Eli Vance almost perceives the All-in-One.
- The Eli Vance was our first collaborator.
- The Eli Vance is indispensable to the liberation.
- Our finest poet describes it thus: Gallum galla gilla ma.
- We are mindful of the words of our greatest philosopher: companum gannennen vorgenot!
- The Combine will pay in kind for their depravity.
- We will put aside Black Mesa... for now.
- Is there reason for delay?
- You must remember us from Black Mesa.
- We see you still in Black Mesa. Clearly we see you in the Nihilanth's chamber.
- We bear witness to the bright eternity of the Nihilanth's demise. You leap, you fall, we see you flash beyond the barriers.
- Ah ghurrrr...
- Lung gah...
- Ga la lung...
- Churr galing chur alla gung...
- Chur lung gong chella gurr...
- Hurrrmmmmmmmmmmmmmmmmm. Durrrrrrrrrrrrrrrrrrrr. Churrrrrrrrrrr---Koff! Hack! Wheeze!
Citizens do not naturally take to vortigaunts.
- Enough of your mumbo jumbo.
- Damn vorts.
- Speak English.
- That's why we put up with you.
- Something must be wrong with me, I almost understood that.
- None of your vort philosophy.
- Stop, you're killing me!
- What did I do to deserve this?
- Stop looking at me like that.
- Some things I just never get used to.
- I don't know how you things have survived as long as you have.
- Sometimes I wonder how I ended up with you.
- If anyone would have told me I'd be pals with a vortigaunt...