The first line includes a Field/Variable and the second one an Auto-Implemented Property, which, as you can read in the MSDN documentation, is just a "quick version" of the conventional Property.
You can see advantages/disadvantages between Properties and Fields in the corresponding MSDN definition of Properties and in links like this one.
Also you have quite a few posts regarding advantages/disadvantages of Auto-Implemented Properties as compared to the traditional version:
These links are for C# but the ideas are easily transferable to VB.NET.
One of these last links refers to another pretty interesting one (also in C#): Properties vs. Public Variables.