THE search engine. This is for discussion about Google as a company, not for discussion of individual sites (SEO, PageRank for a site or "why a site ranks where it does" questions) or Google search results.
Anything related to Google's AdSense program that allows website owners to generate advertising income by running Google ads.
Dedicated to the discussion of Google's PPC system, AdWords.
This is the place for everything regarding Google Sitemaps.
Technical discussion regarding usage of the Google APIs. The Google API developer kits can be downloaded here.
Google's commercial product search engine (once known a Froogle).
Google+
For anything related to Yahoo as a company or their search engine.
This forum is for discussion of Yahoo's contextual advertising system (YPN).
Information about Yahoo's pay per click advertising system can be found here (once known as Overture).
Technical discussion regarding usage of the Yahoo APIs. You can apply for a Yahoo API application ID over here.
For discussion of Microsoft (as a company) and their search engine that powers the MSN search.
This is the place for topics related to MSN adCenter PPC program.
Any search engine without their own forum, can be discussed here.
Directories (such as DMOZ/ODP, Google Directory, Yahoo Directory, etc.) can be discussed here. This is only for sites that are only directories, not sites that have a directory somewhere within them.
If you are a directory owner announcing a new directory (or compiling a list of directories), looking for submissions or a user looking for directories to submit to, this is where the request should go.
For anything regarding the largest human edited directory. Includes submissions, problems, using their RDF data, etc.
Discussion about how to utilize social networks as a marketing tool. This is NOT for requests to join social networks or communities.
This is for forum topics related to Facebook.
General search engine optimization discussion.