The ‘M’ in ‘CPM’ is the Roman numeral for 1000: the industry-standard unit size for purchasing impressions.
Web advertising campaign placement is charged using one of two methods:
Placement is charged based on the number of times people click on an advertisement (to link through to the advertiser’s website).
Placement is charged based on the number of times an advertisement is displayed (even if no-one clicks on the advertisement).
Advertisers buy impressions in blocks. For the Google AdWords network, impressions are bought in blocks of 1000 impressions.
Pricing for impression blocks varies depending on the advertising network. Targeted placement, for example placment on an industry-specific website, usually fetches a higher premium. There may also be a minimum monthly spend.