Themes are the "look and feel" of a site. This includes everything from the font types and sizes used in headers or titles, background and foreground colors, the number and location of regions where blocks can be placed, even right down to the amount of space that exists before and after a paragraph. Everything that relates to style and appearance is controlled by the theme.
The great thing about Drupal themes is that changing the entire look and feel of your site only takes six simple mouse clicks by a site administrator.
Drupal themes are not easy to build, though. The development of a high-functioning, professional looking, and cross-browser compatible theme can also be quite expensive. In an attempt to minimize cost and maximize options, Central Web Services has teamed up with University Relations & Marketing/WebComm to create an interesting and varied collection of exceptionally customizable OSU-centric themes that promote the OSU Brand Identity Guidelines.