SEM is a rather broad term.  It is everything that can be done to utilize the technology of search engines with the goal of promoting a web site and increasing its traffic and increase profits.

Aspects of search engine marketing which we consider to be outside the realm of search engine optimization include the following:

Paid Inclusion

This is simply the practice of paying a search engine or a directory to add a site to its database immediately, rather than setting up that site so that it will be found by the search engine spiders on its own.  In the case of some search engines and directories, paid inclusion is the only way to get listed, for others, it is presented as an option.  If you are willing to pay, your site would be listed sooner.  It is also a useful practice if you wish to make frequent changes to your content, because your site will be spidered more often and you will be able to test how changes affect your ranking.

Traditional Ads

This involves placing paid advertising on the search engine result pages (SERPs).  Normally, these ads appear based on the keywords entered into the search engines and one is charged based on the number of impressions of the ad.  In other words, you pay whether the ad sends anyone to your web site or not.

Pay-Per-Click (PPC) Advertising

PPC ads are simple enough to look at.  They are text-only.  PPC ad campaigns are completely controlled by the advertiser.  You decide which keywords should bring up your ads, you write the copy and you decide how much you want to pay.  As the name indicates, you only pay for an ad when someone clicks it and is brought to your site.  There are two main networks of PPC ads, run by Google and Overture, and each has its own advantages and disadvantages, but in both cases, one should be prepared to spend a great deal of time (and money) monitoring and adjusting such a campaign.

The Advantages of These Practices

In contrast to pure SEO, these SEM practices offer the advantage of immediacy.  If you need to increase your traffic and your visibility right away, and you don't have a problem with spending a lot of money, this may be the way to go.

The Disadvantages

Think about it this way: you have spent a lot of time and money to bring people to your web site. What will they see when they get there?  You have convinced them, at least for the moment, that your site is worth visiting, so they are expecting to find exactly the information they were seeking.  Is it easy to find?  Is it there at all?  You only have a few seconds to convince them that your site can deliver what they want.  Will your visitors fail to find what they are looking for and click their browser's back button to try another site?

Your site must be ready for your visitors.  It needs to be written, structured, and coded in such a way that the information is clearly laid out and easy to find.  If it is not, your site will either be immediately forgotten by visitors, or worse, it will be remembered as one that failed to deliver.