Function Theory [help needed to understand concepts]

Hi, we started function theory which is part of our discrete mathematics course this week.

I'm having little trouble with understanding injective,bijective and surjective concepts....

I dont know why, but sometimes I get confused between how they represent the domain and codomain...

My understanding so far is that injectiv (one-to-one) is a function where its set A images a UNIQUE value in set B... can a horiztonal line test prove that a function is injective ??/

Surjective, I guess is oppostive of innjective, where the set A images 2 values in set B... i.e when x=2 y= +- 4 like there are two values...

and bijective has abit of both ??

If you can list me a good website that explains it with arrow diagrams and sets it would really be great :)

Thanks