Pay per click
Pay you each time a reader clicks on an ad. Usually a few cents per click, though very highly valued keywords can bring $10 or more per click.
Pay per sale
Pay you when a reader clicks through and makes a purchase. Some of these are flat amounts per sale while others may pay a percent commission.
Pay per lead
Pay you when a reader clicks through and fills out a form, provides an email address, etc.
Pay per action
Generic term for any ad that requires the reader to take an action for you to get paid. Includes pay per click, pay per sale and pay per lead.
Pay per impression
Pay you a certain amount each time the ad is served. May only pay once per unique user.
Pay per day/week/month/etc.
Pay you a flat fee for the time period agreed, regardless of traffic fluctuations, click through rates, etc.