Search engine optimization (SEO) is the process of improving the volume or quality of traffic to a web site or a web page (such as a blog) from search engines via "natural" or un-paid ("organic" or "algorithmic").
SEO is creative and it's technical. It's logical, parasitic and opportunistic. The aim of seo is to maximise targeted types of valuable traffic to a website from search engines by improving the visibility of said website in Google organic or unpaid SERPS. Traditionally, this has meant aiming for no1, or top ten positions in Google but SEO has evolved and now a SEO needs to be more a marketer than just a tech geek. A seo needs to be concerned, not just with high positions in Google for relevant keywords, but with analysing competition, traffic and analytics, and converting traffic to leads and sales. I like to draw a line between SEO and Linkbuilding. One (SEO) is on-page (on-site), the other, off-site (Linkbuilding).
SEO is the process and practice of optimizing your website so that it ranks well in search results. When someone types a word or phrase into search engines (like Google) looking for your product, you want to appear on the first or second page of the search results.