The most important thing to remember is that SEO and PPC are complementary strategies. There is no conflict between using the two techniques, and both will be more successful if used together as opposed to using just one or the other in isolation.
The most obvious difference is the price tag that comes with these two techniques. You are paying for the clicks that PPC sends your way, whereas the clicks from SEO are free. Getting to the top of the SEO tree for your most lucrative search terms requires hard work. Getting to the top of the PPC tree for the same search terms requires money (just how much is dependent on the competition for that particular search term) and work. Both PPC and SEO require hard work researching keywords so that you know which search terms to buy (for PPC) and which terms to optimise for (SEO). Different creatives per search term or group (PPC) and different HTML coding for each page of your site (SEO) both need to be written to entice customers. SEO has the added complication that the title tags need to be written with the search engines in mind as well.