The cached files are stored (by default) in the cache/normal/ directory under your Drupal installation directory. The Drupal pages' URL paths are translated into file system names in the following manner:
http://example.com/
=> cache/normal/example.com/_.html
http://example.com/about
=> cache/normal/example.com/about_.html
http://example.com/about/staff
=> cache/normal/example.com/about/staff_.html
http://example.com/node/42
=> cache/normal/example.com/node/42_.html
You'll note that the directory path includes the Drupal site name, enabling support for multi-site Drupal installations.