SEO or Search Engine Optimisation is an organic process of boosting the ranking of your website on the SERP (Search engine results page). It is a gradual process through which your site is optimized according to latest search engine algorithms, and this helps in improving your ranking. With SEO you need not pay for every click on your link on the SERP. But it can take around 5-6 months to see visible improvement in your ranking.
PPC or Pay-per-click is a paid advertising model, wherein you need to pay for every click made on your advertisement on the SERP. The significant benefit that PPC provides over SEO is that it delivers instant results, as soon as you start the campaign and deposit the money, your ads are live. The price of per click is fixed according to the keywords and frequency of search and is at the prerogative of the search engine. Once the balance in your account is exhausted, your ads will stop running. In the long run, PPC campaign should be used in collaboration with SEO to ensure a balanced online advertisement campaign.