global.asa is a text file that sits in your root directory. If it's present, it fires asp/vb subroutines when the application (web server) starts and stops and also when the session (A user comes to your site for the first time in this browser session) starts and stops.
The Application_OnStop and Session_OnStop are not reliable, but you can put any asp code your want into the Application_OnStart and Session_OnStart.
Putting the following code into the Session_OnStart subroutine will direct any new user to index.htm regardless of what uri they follow to your site.
Response.Redirect "/index.htm"
In asp.net, the file is called global.asax and can contain a couple of other subroutines like Page_OnLoad.
Two after thoughts...
A global.asa file can actually go into any directory and will apply to all subdirectories within that tree unless the sub directory has it's own global.asa.
For events in global asa to fire, a user must have session cookies enabled which they need for the forum anyway.
Edited by dpyers