Hi, firstly I've never really understood what injective and surjective means so if someone could give me the gist of that it'd be great!

secondly the question i'm stuck on at the moment is....

let A = {x:x (belongs to) [0, (inf))} and B = {x:x(belongs to) [0, inf)} and the function f: A (tends to) B is given by

f(x) = 1/(2+x^2)

show that f(x) is injective but not surjective

Thanks