Assuming when you say vb you mean vb 6.
Pretty big. The original visual basic does not use the .net runtime environment, and although they have similar names, they are pretty much too different languages. Visual Basic is not fully object oriented, but VB.net is. Error handling is vastly different between the two. VB.Net has try catch blocks where traditional vb uses On Error GOTO statements. These are just a few differences. The list goes on and on.
Here is a link describing the "Visual Fred" Name.
