I'll add to those two responses that it's normally used by men towards men. I do have a friend who occasionally calls his wife 'mate', and I find that indescribably weird. It's a word that women almost never use in the way you describe, at least not here in England.

"How are you" can be used with everyone. Your friends, teachers, even your boss. "How are you mate" can only be used with friends or people you have an informal relationship with. "Mate" is slang for "friend" (used in UK, Australia, New Zealand). Not everyone uses it, but you can if you have a good relationship.

There is no difference. "How are you mate" you use it when you asking your friend, your partner or someone you work with