Google

Established in 1997 Google is now the top name in search, especially if you have a UK website. As far as search engine use is concerned (and these figures keep changing) Google has 78% market share in the UK, with Yahoo at 6% and MSN at 4%. As 85 to 90% of your visitors will come from a search engine it is pretty critical that you achieve a top page ranking with them. Within the site they have several key subdomains that you should familiarise yourself with.
Adsense - tools to enable you to accept PPC ads on your site www.google.co.uk/adsense
Adwords - tools to enable you to target PPC ads on the search engine listings www.google.co.uk/adwords
Google guidelines - what they look for in terms of site design and standards to recognise your site www.google.com/support/webmasters/bin/answer.py?answer=35769