Click-throughs: The number of people who reach your site through a banner.
CPM: Cost per thousand, a common pricing structure that is calculated using a set price per thousand page views for the page the banner appears on.
Hits: Number of files downloaded from the server; each page and each graphic on the page counts as a hit. It's not a very accurate measure of actual traffic to your site.
Page impressions: Also called page views, it's the number of people who see your banner.