Several webmasters and web search engine optimizers were confused by Google's Robots Exclusion Protocol. The latter is the standard for controlling how web pages on a particular site are indexed. Simplier said, it talks about noindex, nofollow, noarchive and nosnippet meta tags.

Earlier, Google released a detailed explanation and examples of how mechanisms work to control access and indexing of websites. The Google Blog has a basic but clear explanation of the robots exclusion protocol.

In a nutshell:

NOINDEX tag tells Google not to index a specific page
NOFOLLOW tag tells Google not to follow the links on a specific page
NOARCHIVE tag tells Google not to store a cached copy of your page
NOSNIPPET tag tells Google not to show a snippet aka description under your Google listing, it will also not show a cached link in the search results

For an in-depth discussion about Robots Exclusion Protocol, click here.