I n short : SEO is not synonymous to JUNK E-MAIL
you can see my sign too for best answer. :)

SEO refers to search engine optimization. It is a technique to improve ranking of your website on search engine.

SEO is Search Engine Optimization. It helps to increase traffic for your website which increases the popularity. The one that comes in the first page of google is SEO and the one that comes highlighted(the very top of search results) is PPC (pay per click).