Search Engine Marketing (SEM) is using Advertising Campaigns and SEO together in order to create more web traffic.  This mixture allows the client to ensure specific pages are targeted through advertisements as well as working on the search results to increase results and track through both means.  This gives the client a better overview of what the ROI could be if one option was used instead of in tandem.  Most major companies have some SEM ongoing at all times; not all are web located.  For example, a store requires you to scan a barcode to produce a coupon that is used either online or in-store.

As SEM is a mixture, no two Marketing Campaigns are alike.  Changing economic conditions, competition, manufacturer price changes, and other details all lead to changes over time.  Goals can also change quarter to quarter.  Holiday shopping is a great example of this.  As online sales peak during the holiday shopping season, SEO is very important when shoppers are searching more gifts and ideas of the next big thing.  Advertising Campaigns may also be used during this time to bring in traffic that otherwise might have stayed on another site.

SEM does not have to start with both types of traffic drivers.  Instead, you can start with an Advertising Campaign and later on establish a SEO drive for your web medium as well.  For more information on click SEO or Advertising Campaigns