Well for one, Visual Basic is the base language that VBA is built upon. You may have guessed that VBA is Visual Basic for Applications.
Typically VB applications need to be compiled into executable form for the program to work, while VBA applications rely upon the hosting program to compile at runtime.
The code syntax is basically the same, and most of the commands, objects, properties, and methods are the same. The main difference is that when using VBA, in AutoCAD for example, you automatically have the AutoCAD objects available in the VBA environment and are able to programmatically access those commands, objects, properties and methods, however in VB you must explicitly tell the VB compiler to add a reference to the "object model" for the application you want to interact with.
Forms and controls are a little different as well. The VBA forms when used in VB are referred to as "designers" and VB forms per se' cannot be used in VBA, you can import the VB form controls to VBA and utilize them, but since many of the controls have the same designation, sometimes there are problems, but the controls are effectively the same, only the VB controls have a little more functionality.
There are some other differences, but we could spend all day talking about the differences. They are essentially the same...if you can code VBA pretty good then you can code VB as well.