A structure group is a container for pages and sub structure groups. It is similar to a folder in the file system. On the presentation server a Structure Group corresponds with an actual folder on the file system. It is possible to add metadata to eacht folder.
The difference between a folder and a structure group is that a folder contains components and a structure group contains pages. The fact that a file system folder in the structure of the website corresponds with a Tridion structure group, and not a Tridion folder, is somewhat confusing.