What exactly is a theme?
Just as structured programming revolutionized system building, modern web sites have strived for a conceptual structure. Today we try as far as possible to separate information, process and presentation. Think of the three as a venn diagram of overlapping circles: content, function, and theme. 

Images and text are information, and the core and modules necessary to work with them are function.

The theme is the look and feel, the overlap is your information architecture. In Drupal, you can retain the same function and information, and completely change the presentation by changing the theme.

Think of it as structured programming taken to the next level.