They are different.
SEO is what do to your web site to gain, what is called, Organic Traffic (people find you through the search engines). You are optimizing your site using the META tags on each page (title, description, and keywords) and sprinkling your keywords throughout your page.
PPC (Pay Per Click) is an avenue for instant traffic. You're working through Google Adsense, the Yahoo Search Network, and/or Microsoft's Adcenter to bid on keywords to drive certain traffic to your site.
PPC is harder to master than SEO, but will give you quicker results.
Also, SEO is for general web sites, while PPC is specifically to make sales via landing pages, squeeze pages, etc. You want to get a good ROI (Return on Investment) when you're spending your hard-earned dollars on PPC.
Hope that sheds some light for you.
Regards,
Tony Thomas