.NET framework is an API
In short, an API is a program that doesn't do anything on it's own, but allows you to write other programs more easily.
For example, a programmer can write his own to code to do common stuff like formatting the date, displaying a dialog box, etc.
But it would be much
easier to write one generic API, and have everyone use that.
I'm not sure which update you're seeing, there are several related to .NET:
o If it's an UPDATE then you'll want it, it will probably fix security and stability issues.
o If it's the .NET package then you don't need it, but it would not harm installing it (Takes a while though).
UNIX was not designed to stop you from doing stupid things, because that would also stop you from doing clever things.