Like bloodline said, the Visual Basic language by itself is just another BASIC, but I gather you're more interested in the "for Applications" part of the abbreviation. The Microsoft Office and, more specifically, Microsoft Excel object models are well documented, and most sections of the documentation include sample code.
The "Microsoft Office Developer Center" should have all the information you need: <
http://msdn.microsoft.com/office/>. If you find yourself wanting more, I recommend reading up on .NET (if you're working with Office 2003) or COM (for all other versions).
Trev