See SEO and PPC are two different ways of promoting a website . in seo as we know we list our website in various search engines and we look after the site ranking in the search engine, by maintaining our site by looks , info, content, performance etc. and other things like submitting links of our site to other related sites. but in PPC which is called pay per click we pay for the as many as clicks, our site viewers click the site, and we pay for the ads we made in other sites, this is also a way of promoting the site, other ways are blogging , writing article about our site, joining forums, etc.