"if they say the Website is Linux or Windows-based, what does that mean? In either one, don't I just upload my HTML files into a folder that can be accessed (but not altered) by both Windows and Linux users?"

Linux will sometimes be cheaper, reflecting that there are no licensing costs and (by some accounts) lower maintenance costs too.

If you are simply uploading HTML, you'll see no difference. If you are doing server side scripting in PHP and using a MySQL database, you can still do that with either system as those packages are multi-platform and you will again see little or no difference.

But there are some pieces of server-side software that only run on Windows. If you really want to use ASP or Microsoft SQL for example, you'll have to use Windows. Windows server also supports Microsoft Front Page extensions better, if you want to use that.

Anahata