Various similar terms

While browsing through the web for proper understanding of the semantics of various online marketing terms this is what I learnt:

Cost per milli=Cost per 1000= Cost per impression:
These three terms all mean the same. This model of online marketing generates revenue in the ways similar to the traditional marketing- amount of exposure. The firm is charged on the basis of number of people to whom the content was displayed. Mind it DISPLAYED.

Cost per click: Is the next step in online marketing evolution. Here the firm is charged either a fixed or a varying amount for every click on the ad being displayed. This is a better quantifying metric to estimate the success of ones marketing campaign as you can actually see how well you ad campaign is doing and accordingly change it.

Cost per action: This is the final step of evolution in the online marketing strategy. Here is the firm is charged on the basis of execution of a pre-defined action like purchase of a product, filling of a form etc. This ad campaign though best for the advertising firm is not the best from the website's point of view. because majority of the people still buy things offline (for reasons click here- http://www.theatlanticwire.com/technology/2011/12/we-still-shop-mall-bunch-reasons/45738/ ). People come online survey the product of their choice and still buy stuff from the store/mall.

Lastly we have another form of marketing campaign but this one is different in its essence. Pay per sale followed by Groupon and similar businesses needs equal dedicated efforts from both sides.