SEM or Search engine marketing is a term that includes a few aspects.
The first aspect is PPC or pay per click campaigns. pay per click means that we make an advertising campaign that is fit for your budget. we than research keywords relative to your product or service, we make you a few ads and after you review our strategy we run your campaign. from this moment on your ads will start showing on Google or yahoo until your daily budget is finished. on the next day your ads will automatically start showing again until your budget is finished.
Another form of SEM if via social media, this means writing on blogs, forums, facebook and more.
You are probably noticing the buttons on the bottom of this page, what they do is help you tell your friends about this page via facebook, tweeter and other Web2.0 platforms, this is a form of SEM as well.