Worm_Autorun.BSG may be downloaded from remote sites by other malware. It may be downloaded unknowingly by a user when visiting malicious Web sites.

It drops copies of itself in all physical drives. It drops copies of itself in all removable drives. It drops an AUTORUN.INF file to automatically execute dropped copies when the drives are accessed.

It connects to certain URLs to download possibly files. As a result, malicious routines of the downloaded files may be exhibited on the affected system.

Technical details can be found at this Trend Micro page.