CPM stands for cost per thousand, referring to the number of people who see an ad. Media buyers use this standard to compare advertising options since reader, viewer and listener numbers and rates vary so much. With print publications using circulation numbers and broadcast mediums typically using audience numbers, CPM helps create an "apples to apples" analysis. In case you don't want to do the math yourself, there are a number of websites with automated calculators to do it for you, like
this one from SRDS. SRDS even breaks out the formula by media type with multiple variables (it's basically the same formula but just more specific in type and source), such as:
For print media (when audience data is not available):
CPM = Cost of 1 ad x 1000
Circulation
For broadcast media (based on homes reached by a given program or time period):
CPM = Cost of 1 unit of time (commercial) x 1000
Number of homes reached by
a given program or time period
Media costs, as in what you pay to actually have your ad appear in the newspaper or on TV, get the largest allocation in most ad budgets, so it's definitely worth your time to calculate the CPM and let it play a role in your strategy. Of course, like all aspects of advertising, you shouldn't use it in isolation either. Many other variables play into the quality of that number. For example, a 2x2 ad will have a much lower CPM than a half page ad, but that doesn't mean the 2x2 is a more effective use of your money. Likewise, if your TV commercial reaches people who are too far away to actually drive to your business, it won't be an "apples to apples" comparison with a newsletter for a neighborhood association down the block from you.