The Autopsy Forensic Browser is a graphical interface to the The Sleuth Kit and other digital investigation tools. Together, they can analyze Windows and UNIX disks and file systems (NTFS, FAT, UFS1/2, Ext2/3, etc.).
Autopsy 3 is Java-based and designed to be an end-to-end platform for digital forensics. That means that it can be extended by other developers. Full details can be found on the sleuthkit.org site.
Autopsy 2 is an old-school HTML-based interface. You can get the list of official features at the sleuthkit.org site.