The .adm file allows administrators to configure the security setting for scripts. The options are 'Allow only signed scripts', 'Allow all local scripts and signed remote scripts' (a remote script is one downloaded from a website, although I'm not sure how this is determined - perhaps using the Attachment Security APIs?), 'Allow all scripts' and 'Disabled', which prevents scripts from running at all. The latter is the default.

You can find a discussion of the security policy at http://blogs.msdn.com/powershell/archive/2006/08/03/687838.aspx.