search engine optimization is what you do on your website to enable the spiders to crawl it easily, rank you higher and improve your website visibility FREE of charge. Pay per click, is when you pay money for each click made. Optimization is needed for these adverts though, in terms of content, relevancy and also you need to make sure the page the user is forwarded to is relevant and will generate sales for you.

SEO and PPC are so similar, that they both work hard to get a customer to the site, and make sure they generate a sale. But one is Free and the other is paid. although some people argue that SEO isn't really free bz you end up paying to employ someone to do SEO for you ;)