Rod,
In simple words the .NET Framework is a piece of software necessary to run
programs that are written specifically for it. If you don't have the .NET
Framework installed on your machine, you won't be able to run these
programs.
How would you know if a program is written for the .NET Framework? Usually
the installation procedure of this program will warn you, install it for
you, or the program documentation would mention it as system requirement.
There's no compelling reason for you to install the .NET Framework if you
don't have such programs, but if you have a fast Internet connection you can
go ahead and install it.
If you want to read some more about it, start from
http://www.microsoft.com/net/basics/
Gabriele
