It is an object oriented programming language. it contains a systematic collection of classes and objects.
please read Object oriented term better.
Visual Basic .NET comes with thousands of built-in classes which are ready to be used.
So it is easier to write in VB.net then Visual Basic. once we have learn how to use .net framework.
Creative Calligrapher dot com has answered every thing. But the Question of Why we use it lies open. I will try to fill in the Gap.

If you are appearing in a VB exam you will use it , if you are interested in earning money you will use it, if you are interested in just programing you will use it, if you are a VB programmer you will use it, if you have a lot of time in your hand you will use it, now the main reason if you like the kick of success (virtually) you would use it. VB is a relatively easy language to learn and sovling tricky problems gives a feeling of success (quick and deserving success), so just for the sake of it you can use it