VB.net is the latest microsoft iteration of Visual Basic.
The differences between VB 6 and VB.net in some cases are subtle and can trip up a seasoned VB programmer. Particularly overloading, true object inheritence, strict type checking, etc. It uses what is called the Dot Net framework which is a common libary that can be interchanged with VB, VC, etc.
VB.net is not backwards compatible with VB6 unless it is a relatively simple code. MS does provide a migration tool, but as Im sure others will attest, its basically useless. It generates a to-do list that is so long, you might as wel re-code the application.