"Mate" is used between good friends, especially in the UK or Australia. It's colloquial. "Friend" is just the basic word. Be wary about relying on a dictionary for "mate", as you may end up with the National Geographic meaning. :D

Nothing, really. Although the meaning of the word "mate" varies in different countries. In Oz it means "friend". In England it is someone in the navy. In America I don't have a clue.

Friend = the 'neutral' word to describe someone you're close to. Mate = a more informal word for 'friend' (especially among men) and also a term of address for anyone the speaker is on good terms with (in the Commonwealth). It's not uncommon for you to go into a shop and be addressed as 'mate', especially if you're a young man. It's just a friendly way of addressing someone.

Mate is a close friend. It comes from British navy

Mate is husband or wife. Isn't?