Windows, Linux is just a matter of preference really. There are differences but mainly boils down to choice. Some prefer windows, some linux (I prefer linux)
Shared is for
hosting a web site and you share an ip address with many other.
Reseller is for
hosting many sites, or selling plans a bit like webhosting do.
I believe
VPS has had the most downtime. This type of account is similar to reseller as in it can
host many different sites. The difference is that rather than get a portion of the hard drive, you get a petition (hard drive broken up into seperate blocks) of the drive, so you have your own operating system, apache server, and have root access.