S.E.O is a process of optimizing your website for search engines, it can be further categorized into onpage & offpage
when you deal with meta tags, title, headings, internal link structure of website etc then it refers to as onpage optimization
on the other hand, when you deal with blog commenting, directory & article submission etc for the sake of getting a backlink
to boost your rankings then it refers to as off page optimization!
PPC on the other hand is website advertising model, in which you only pays for the clicks made and not for
the impressions for your banner/text ads on different publisher websites