The SAM key contains the user account database. The information is encrypted and stored in the %systemroot%\system32\config\sam file.
This is the definition from a Windows magazine article:
HKLM\SAM holds local and domain account information, such as user passwords, group definitions, and domain associations. By default, this key is unreadable by even the system administrator account. Looking inside HKLM\SAM is not very revealing because the data is undocumented and the passwords are encrypted with a one-way mapping (e.g., you cannot determine a password from its encrypted form).
Article URL: <a target="_blank" href=http://www.winntmag.com/Articles/Index.cfm?ArticleID=122&pg=2>http://www.winntmag.com/Articles/Index.cfm?ArticleID=122&pg=2</a>
Cheers
Andy