I started with this list. Far from comprehensive, but a good start (and off the top of my head).

• SEO - Search Engine Optimization - Fascinating to me.  Search engines at least, and this field is a peripheral hanger-on to the big market which search engines have created.  Essentially SEO is making your web site so that it will increase traffic from search engines to your site.
• SEM - Search Engine Marketing
• eCPM - The approximate amount of money you’ll make for each thousand visitors to your site.
• ROI - Return on Investement.
• CTR - Click-through rate - How frequently people are clicking on your ads or links or whatever.
• PPC - Pay Per Click - types of ads where you pay only when people click on your ads.

This is the list of popularity rankings:

1. ppc advertising
2. roi
3. seo
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7. ppc search engines
8. roi calculations
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