- CPM : Cost per impression. It means you'll earn the indicated amount each time you show the advertisers' page/link/banner. Usually set to one thousand views.

- CPC : Cost per click. Here viewing is not enough : you'll earn if and only when your visitors click the link/banner and visit the site.

- CPA : Cost per action. The advertiser will pay you if your visitor provide some action while viewing the pages.

- CPL : Cost per lead. You'll be paid if the visitor fills a form for subscribing to a newsletter or a site.

- CPS : Cost per sale. Means what it says, no ?! The best paid but ... the hardest to provide ...

- Unique Impressions / Clicks : A unique impression is when a banner is displayed on your web site. This does not mean that someone has, or will click on the banner, just that it was displayed on your page.
A unique click is when someone clicks (repeated clicks from one individual does not count) on the banner you have displayed.
If a visitor to your site sees the same banner in less than 24 hours either by re-visiting your site, or by refreshing/reloading page then the impression's do not count, only non unique impressions are generated. The time is tracked from the last unique impression timestamp so non-unique impressions do not reset the time mark again. After 24 hours the next unique impression may be generated from same visitor.