Read from iceman's reference and lets move on. An impression means a viewed ad, not a click through.
Banner ads are generally sold on the basis of CPM, cost per thousand impressions. $20 CPM is typical but you can find rate cards asking $100 CPM or more or as low as $10 CPM. Also note that rates quoted on rate cards are usually negotiated down.
Generally, when you buy banner ad space, you buy it in a chunk where the seller guarantees you a minimum number of impressions. For example, they may show your ad for a week and guarantee at least 150,000 impressions for which you'll pay the CPM x 150. Others may count up the number of impressions throughout the week, then send you a bill based on the CPM. Most advertisers want to work with a fixed maximum cost.
News 
Market Data 
Discover 
AI
