When requesting banner campaigns, you are given the option to pay on CPC or CPM basis. CPC is paid per click and CPM is paid per 1000 impressions (views).
As a general rule, if you have attractive banners (1-2% CTR) you generally will want to use CPM to avoid paying for each click. On the other hand, if your banners are less attractive (less than 1% CTR) you will want to use CPC to ensure you are paying only for clicks and not burning through your campaign budget.
If you are unsure what type of campaign to run, CPC or CPM, please contact Live Support or your Account Manager.