locked: one use for a specific carrier

unlokc: any carrier

My name is Thomas, I'm Italian and I'm in Manchester since 1 month.
Like in Italy I'm searching for a good "local" forum to know the phone market because I'm a geek (lol) then I find you by Google.

Before I can put here stupid post someone can explain me the "locked, unlocked phone"?

If I buy a phone... Example a Samsung GN2 or a Htc X BRANDED
1st question: unlock a phone = lose warranty?
2nd question: If i give it to a shop, it's look like a modding (like cyanogen mod)? Or they put a unlocked UK "original" software?

I know this is only a post of presentation but this questions is too simply to make a new thread for the native UK.