As mentioned in the previous answer, CPM stands for cost per mille (and mille is Latin for 'thousand'), which means that you sell your ads on a per thousand ad impressions basis.
An example CPM order from an advertiser could look like this: 100k impressions of a 300x250 medium rectangle ad that will run from 5/1/2011 to 5/31/2011. If your CPM is price for this ad is $5, then this would cost the advertiser $500 and their ad would be displayed 100k times evenly between 5/1/2011 and 5/31/2011.
Tenancy is defined as being occupied for an amount of time. This means that if you choose to sell your ads based on tenancy instead of CPM, you're going to be selling them for a specific price for a specific amount of time. For instance, you can set up a 300x250 medium rectangle and sell it for a specific price per week and when an advertiser buys the ad they receive 100% of the impressions during that time period. An example tenancy order from an advertiser would look like this: a 300x250 medium rectangle ad for 1 week for $250 that starts on 5/1/2011. For a tenancy campaign, the advertiser doesn't need to pick an end date since the ad will be displayed on 100% of the ad impressions from the start date they selected.