CPA
Cost Per Action or CPA (sometimes known as Pay Per Action or PPA) is an online advertising pricing model, where the advertiser pays for each specified action (a purchase, a form submission, and so on) linked to the advertisement.
Direct response advertisers consider CPA the optimal way to buy online advertising, as an advertiser only pays for the ad when the desired action has occurred. An action can be a product being purchased, a form being filled, etc. The desired action to be performed is determined by the advertiser.
The CPA can be determined by different factors, depending where the online advertising inventory is being purchased.

ROI
In finance, rate of return (ROR), also known as return on investment (ROI), rate of profit or sometimes just return, is the ratio of money gained or lost (whether realized or unrealized) on an investment relative to the amount of money invested. The amount of money gained or lost may be referred to as interest, profit/loss, gain/loss, or net income/loss. The money invested may be referred to as the asset, capital, principal, or the cost basis of the investment. ROI is usually expressed as a percentage rather than a fraction.

CPC
Cost per click (CPC) is the amount of money an advertiser pays search engines and other Internet publishers for a single click on its advertisement that brings one visitor to its website.

CPM
CPM is frequently used in advertising to represent cost per thousand (where M is the roman numeral of 1000). When used in online advertising it relates to the cost per thousand page impressions. CPM is usually referred to the number of clicks registered in a website by the readers. Ad Networks, such as AdSense, calculate ad revenue for websites based on CPM.

(From Wikipedia)