An excellent video by Matt Cutts of the Web Spam Team at Google. He explains the basic of how Google search works including page rank, relevance, reputation, keywords, importance of posting daily and understanding and communicating your readers.
The video is really comprehensive and keeps getter better. It is well worth viewing. I would recommend to anyone.
Check it out:
http://wordpress.tv/2009/05/30/matt-cutts-google-sf09/