Hi Alan,
Essentially, Apache is a Linux server O/S. Its just the most common one in use (Red Hat being another). Linux are UNIX based operating systems and are just built differently to Microsoft systems and are well suited to servers as they just very CPU efficient and run and run without needing to shut down or reboot for minor admin tasks. There is a comparison
here from an independent hosting company. Essentially, there isnt a lot in it, but Linux have the edge in terms of stability, security and longevity in the business.
The majority of hosts (Pipe10, FastHost, 1&1, JodoHost, etc) will give you a choice of Windows or Linux (usually Windows Server 2k3 or Apache/Red Hat). You just need to work out how much you want to spend and what sort of disc space you need. It doesn't matter what you have created the website in, FrontPage (eugh!), DreamWeaver, etc. It can be hosted on either a Windows or Linux server.
Thanks,
Sarah