It could be a Web site which you can build using ASP.NET. In this case, the client site will be represented by a Web browser on any platform. Making a custom server and client software using .NET would be easier, but the clients will work only on the platforms where you can install .NET. Also, if your client is written very accurately based on
System.Windows.Forms
, you can run it on Windows with .NET but also on many other platforms using Mono, see
http://en.wikipedia.org/wiki/Mono_%28software%29[
^],
http://www.mono-project.com/[
^].
So, it presents a certain trade-off between the easier way and more universal way. You will need to analyze your resources, required features to decide on feasibility and expected quality of each of the two approached.
—SA