Search engine optimization (SEO) is the process of increasing the quality and quantity of website traffic by increasing the visibility of a website or a web page to users of a web search engine. SEO refers to the improvement of unpaid results (known as "natural" or "organic" results) and excludes direct traffic/visitors and the purchase of paid placement. SEO may target different kinds of searches, including image search, video search, academic search,[2] news search, and industry-specific vertical search engines.
Source: Search engine optimization on Wikipedia

Which is more important: content or links?

Matt Cutts, Google Software Engineer

Does Google use the "keywords" meta tag?

Google does not use the keywords meta tag in web ranking.
Matt Cutts, Google Software Engineer

Are CSS-based layouts better than tables for SEO?

Matt Cutts, Google Software Engineer