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 2 is an old-school HTML-based interface. You can get the list of official features at the sleuthkit.org site.
If you want more information on using Autopsy to conduct an investigation, refer to the Autopsy User's Guide.
If you want more information on contributing to Autopsy and developing for it, refer to the Autopsy Developer's Guide. Note that Autopsy 3 was designed as platform to integrate other digital forensics tools. The developer's guide also outlines how to develop modules to integrate into Autopsy.