Cost Per Impression (more commonly referred to as CPM) refers to ads that are valued based on the number of people who see them. (The “M” stands for the Latin mille which refers to the standard 1,000 impressions that your costs will be based on.) This is the opposite of pay-per-click (PPC). Typically, CPM is most appropriate if you are trying to raise awareness or visibility for something. You don’t care so much if someone clicks on an ad, so long as they see it and remember it.
There are a lot of reasons why CPM is not as common as CPC in digital advertising, but the primary reason is that CPC is far more accountable. Many people consider a click to be more meaningful than an impression. A click requires a conscious decision most of the time, and in most situations, after someone clicks, you can then analyze subsequent actions that they took on your website. You can’t do that based only on impressions. In any case, there are some situations where CPM is the best way to go (TV, print ads, etc.), but most of the time in AdWords you are going to focus on clicks, not impressions.