Difference Between SEO and SEM?

friend what is the exact difference between these two topics : Seo and Sem ? :eek::eek::eek:

[font=verdana]This is a topic that has been discussed many, many times before. Please have a look through past threads or use the [url=http://www.sitepoint.com/forums/search.php]search function to read what has already been said. If you have any further questions after that that haven’t been answered then feel free to either start a new thread, or add them to an existing thread if that’s appropriate.