In Autopsy, a "case" is a container concept for a set of images. The set of images could be from multiple drives in a single computer or from multiple computers. When you make a case, it will create a directory to hold all of the information. The directory will contain a configuration file, some databases, and some other information. The configuration file as a .aut extension.
If you want to view case details or edit some case information, use the Case Properties window.
Refer to the Creating a Case page for more details.
To open a case, choose "Open Case" from the File menu or use the "Ctrl + O" keyboard short cut. Navigate to the case directory and select the ".aut" file.