What is bilinear map in cryptography

Question

hi friends any one know what is bilinear map in cryptography.
A bilinear map is a map e : G × G ? GT , where G is a Gap
Diffie-Hellman (GDH) group and GT is another multiplicative cyclic group of
prime order p with the following properties [16]: (i) Computable: there exists an
efficiently computable algorithm for computing e; (ii) Bilinear: for all h1, h2 ? G
and a, b ? Zp, e(ha1, hb2) = e(h1, h2)ab; (iii) Non-degenerate: e(g, g) 6= 1, where g
is a generator of G.

 
what is this?I am not getting I have lots of search on internet but not get concept please give an example so that i will get understand concept.please help me thanks