What are SEO and SEM?
What's the Difference Between the Two?

SEO stands for Search Engine Optimization.  SEM stands for Search Engine Marketing.    

Search Engine Marketing (SEM) consists of a variety of tools used to improve the visibility of a web site within the results pages of the search engines.   

The three main areas of Search Engine Marketing are: 

Search Engine Optimization (SEO)

Pay Per Click (PPC) Search Engine Advertising

Pay for Inclusion 

Search Engine Optimization emphasizes such things as Meta Tags, navigation structure, internal and external links as well as content writing and management. 

Since the goal of Search Engines is to respond to a user query with the most useful and relevant web pages search engine optimizing has as much to do with the visitor experience as it does the search engines.   

For instance since Meta Tags do not affect the viewing experience of a web page it could be said that these are created for the search engines.  However, since the search engines display the information given in the Meta Tags in their results even these affect the user experience. 

Google, Yahoo and MSN all offer a Pay Per Click program.  With these paid for results your site can rank on the first page of one of these search engines within a matter of days.  These programs are based on an auction type concept.  Search terms (keyword phrases) are bid on and your site’s ranking is based on a combination of factors including your bid amount.  Basically the larger the bid the better the placement.  

Since Search Engine Optimization (SEO) can take a considerable amount of time and is not completely predictable many web site owners make use of Pay Per Click advertising in conjunction with the SEO strategies.  A dual use of SEO and PPC can provide far better visibility than the use of either alone.

Many search engines including Yahoo offer a Pay for Inclusion program by which you can pay to have your web site spidered (or crawled) more quickly.  These do not guarantee inclusion or ranking.