SEO is the process of increasing traffic to a website from search engines like Google, Yahoo and through organic and Pay Per Click search results for targeted keywords.

PPC is an online advertising model where advertisers pay a certain amount of money each time someone clicks on an ad which takes him or her to the advertiser's website.

SEM is the act of trying to promote websites by increasing their visibility in search engine result pages (SERPs). SEM methods may include: search SEO, PPC, and paid inclusion.(Wikipedia)