Help me with I"postback" and "pixel" CPA marketing online -- 2

I am going to advertise my html banners with a company. (CPA). So I will pay them everytime someone registers thru my banners and only then. My banners are affilliate based. And I get them from a company that I am an affilliate for. The company where I will advertise my banners at, asked me:How will we check the number of registrations? They then told me thay were asking for "postback" or "pixel". So I guess I need postback to track/ be able to send them the evidence on registrations? Please help me fix this big problem asap! This is very urgent.