The humor of the joke

[NewHope]

It might be caused by my culture. I'd like to know why the joke below is funny:

A man walked in to a bar after a long day at work. As he began to drink his beer, he heard a voice say seductively "You've got great hair!" The man looked around but couldn't see where the voice was coming from, so he went back to his beer.
A minute later, he heard the same soft voice say "You're a handsome man!" The man looked around, but still couldn't see where the voice was coming from.

When he went back to his beer, the voice said again "What a stud you are!" The man was so baffled by this that he asked the bartender what was going on.

The bartender said "Oh, it's the nuts--they're complimentary."

By the way, what does "stud" mean?

 

[Francois]

In bars you are sometimes given some nuts, peanuts or pistachios etc. when you order a drink.
Complimentary can mean free and making compliments ;-)
A stud is a horse used for reproduction; by extension, it's a great lover, or "performer"; well, you get the idea ;-)

FRC

 

[NewHope]

So, both "nuts" and 'complimentary" are puns.

 

[Tdol]

Free complimentary nuts? ;-)