Search Engine Marketing, or SEM, is a form of Internet Marketing that seeks to promote websites to search engine users by increasing the website’s profile so that it will appear as high as possible in the results list (also referred to as the Search Engine result pages (SERPs).

SEM methods include: Search Engine Optimization (or SEO)
paid placement (or PPC)
paid inclusion (you have to pay to get into the search engine)
allowing search engines full access to your content for free as a way of enticing buyers

What are SERPs?

Search Engine Results Pages

The links that the search engine delivers when you type in a query, for example American Cocker Spaniel. 

The whole goal of SEO is to get to the top of the SERPs.

SERPs will also frequently have ads on them, on the tops and right hand side of the page. This is an essential part of SEM, Search Engine Marketing, and part of a PPC Pay Per Click strategy to drive traffic, in addition to using SEO to try to drive traffic.

SEO is a more long term strategy than SEM and PPC.