What IS 'love'?

i would say an emotion like no other- like, you are not just wanted, but someone actually lives for you being around, care about you deeply, will do anything for you, worry about you when you're not around, constantly thinks about you, etc etc

How do you know if you are receiving it?

as yellowted said, you feel safe, and cared about, you feel their's meaning

How do you give it?

kisses, hugs, good deeds, words " i love you", nice gifts (flowers, chocolate etc)