What's the difference between flirting and complementing?

You can give a compliment without having any romantic intentions to that person while flirting suggests your trying to take things further.

A deserved compliment is just that with no agenda. When I flirt I plan to get them into my bed

Way too hard to explain

Intention

its flirting if she's attracted to you, sexual harassment otherwise

You can compliment someone without any romantic intent whatsoever. Flirting by definition implies romance.