Dust-D,
As far as searching for CPA, it probably didn't come up with any results because the abbreviation is too common or something like that.
I agree with Rob_TID that just reading up on some of the big threads should be the #1 priority. But since you probably won't find a description of that abbreviation in those threads, I'm gonna help you out;
CPA = Cost Per Action. This refers to any type of affiliate offer where you as the affiliate gets paid a certain amount of $ for each sale (or "Action") that is made. CPA offers usually requires the user to BUY something, like an ebook, a physical product, or a monthly service.
CPA payouts can vary from $x.xx up to whatever based on percentages of sale.
CPL = Cost Per Lead. These are offers where the users most often do not need to pay anything upfront (other than sometimes shipping&handling) for limited-time free offers or trial samples of products and services. You still get paid for the "Lead" that you were able to bring to the merchant. Meaning, you provide information about the user such as name, adress, phone number, zipcode, email, etc. and you get paid $ for that lead.
CPL payouts can be very high, but are usually a fixed $ amount, not a percentage.
CPM = Cost Per Mille (thousand) impressions. These are offers where you get paid for every time a banner / ad is displayed on your website. For every 1000th time that banner is displayed, you get paid a certain amount of $ (usually in the low $x.xx range).