CPM = cost per thousand where the M is the Roman numeral for a thousand. When you're talking about ad rates, CPM refers to the amount you'll get paid for every 1,000 downloads when you're running a CPM-based campaign. Example: You run an ad with a $30 CPM. During the campaign, you get 5,000 downloads. 5,000 divided by 1,000 = 5 multiple by the $30 CPM rate = a $150 payout. This is what you'd get paid, regardless of how well the ad campaign did in terms of actual sales conversion for the sponsor. The downside for the sponsor is, they spend the money whether or not the campaign earns them any new sales. The upside for the podcaster is, you get paid no matter how well the campaign actually performs.
CPA = cost per acquisition. These types of campaigns pay a one-time fee for every customer you acquire for the sponsor. Download numbers are mostly irrelevant for CPA campaigns, as the sponsor isn't paying based on that. Instead, they're paying every time a customer goes to their site and makes a purchase, usually using a special link from your website or a special promo code. It's very similar to an affiliate program. You send your listeners to your sponsor's website. They make a purchase, you get a payout from the sponsor. The downside is, if no one follows thru on the campaign, you don't get paid. The upside is, the sponsor can test the waters with your audience at no cost to them.
Please let me know if you have any questions.