Classes

Although I've been involved with C# since the version 1.0 beta, I've never seen the term 'lightweight' class defined.

Microsoft themselves appear to use it to mean a 'small' class where you might otherwise have used a struct, but needed reference type rather than value type semantics for some reason.

Check this link for example:

http://msdn.microsoft.com/en-us/library/bb383979(v=vs.100).aspx