^^ Cost-Per-Acquisition & Cost-Per-Click CPA - You get paid based upon each sale or lead you make CPC - You get paid for each time someone clicks on a link I'd recommend NeverBlueAds. The fact being that you get paid more through CPA's then CPC's.
I hate adsense and haven't used it in along time as u need good traffic and high clicks OR high paying keywords. That said, I too have just been accepted to neverblue so hopefully I'll be able to answer that question sooner, rather than later.
Lead is like someone who may be converted to sale (so if a salesman offers you about a product, and you decide to get more information about it, there's a high chance that you are interested and can be "profited" ) I haven't got any $ yet from neverblue..do you have to put a post and advertise the ads ? Coz I simply put them in a rotating banner and hope someone clicks and register something