Here are some acronyms we may refer to:

P4P - Pay For Performance / Pay For Placement - This is keyword bidding for the advertising spots available on a SERP.   If your bid is high enough for a particular keyword, then your ad will show up in the "Sponsored Listings" section of the results page.   In general, you only pay when a user actually clicks on your advertisement link. There are many complex bidding tools and strategies for P4P.   Examples are Yahoo! Sponsored Search (formerly Overture) and Google AdWords

PI - Paid Inclusion -   A search marketing product unique to Yahoo! that integrates paid listings into the natural results of a search engine.

PPC - Pay Per Click - Specifically refers to the payment method - per click.   For instance, you may pay $.15 / click or $.15 for every visitor to a target site.   More generally, PPC refers to pay-per-click marketing products like Paid Inclusion (PI).

SEM - Search Engine Marketing - Marketing of a brand, product, service or website via major or minor search engines.   Generically refers to the industry around advertising in one way or another via the search engines.

SEO - Search Engine Optimization - The process of integrating intelligent design and copy into a website with the objective of ranking well in a search engine's "organic" results pages.   In other words, designing your site to rank well in the major (and or minor) search engines.

SERP - Search Engine Results Page - The results page that is displayed when a user types a query into a search engine.   Usually referring to the natural or organic results set but could also include the P4P listings.

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