Actually VBA can be used to compile DLLs. The Office 2000 and Office XP Developer editions included a VBA editor that could be used for making DLLs for use as COM Addins.
This functionality was removed in later versions (2003 and 2007) with the advent of the VSTO (VS Tools for Office) software, although obviously you could still create COM addins in a similar fashion without the use of VSTO (or VS.Net) by using VB6 IDE.
VB (Visual Basic only up to 6.0) is a superset of VBA (Visual Basic for Applications). I know that others have sort of eluded to this but my understanding is that the semantics (i.e. the vocabulary) of VBA is included in VB6 (except for objects specific to Office products), therefore, VBA is a subset of VB6. The syntax (i.e. the order in which the words are written) is exactly the same in VBA as it would be in VB6, but the difference is the objects available to VBA or VB6 are different because they have different purposes. Specifically VBA's purpose is to programatically automate tasks that can be done in MS Office, whereas VB6's purpose is to create standard EXE, ActiveX Controls, ActiveX DLLs and ActiveX EXEs which can either work stand alone or in other programs such as MS Office or Windows.
Do you want compare VBA with VB-Classic (VB6..) or VB.NET?
VBA (Visual Basic for Applications) is a vb-classic-based script language embedded in Microsoft Office applications. I think it's language features are similar to those of VB5 (it just lacks some few builtin functions), but:
You have access to the office document you wrote the VBA-script for and so you can e.g.
Example: Set the value of an excel-cell
ActiveSheet.Cells("A1").Value = "Foo"
VBC and -.NET are no script languages. You use them to write standalone-applications with separate IDE's which you can't do with VBA (VBA-scripts just "exist" in Office)
VBA has nothing to do with VB.NET (they just have a similar syntax).
VBA stands for Visual Basic For Applications and its a Visual Basic implementation intended to be used in the Office Suite.
The difference between them is that VBA is embedded inside Office documents (its an Office feature). VB is the ide/language for developing applications.
VB is not a language. VB is a program that hosts VBA, just as Office hosts VBA. VB is a set of App objects, just like Word and Excel have, and a forms package, just like in Office.
So you can only write VBA code in VB.
PS this info is on the INFO tab on the VB question page for VB.
From VBA Info
VBA 6, was shipped in 1998 and includes a myriad of licensed hosts, among them: Office 2000 - 2010, AutoCAD, PI Processbook, and the stand-alone Visual Basic 6.0
Here's a more technical and thorough answer to an old question: Visual Basic for Applications (VBA) and Visual Basic (pre-.NET) are not just similar languages, they are the same language. Specifically:
In an old VB reference book I came across last year, the author (Paul Lomax) even asserted that 'VBA' has always been the name of the language itself, whether used in stand-alone applications or in embedded contexts (such as MS Office):
"Before we go any further, let's just clarify on fundamental point. Visual Basic for Applications (VBA) is the language used to program in Visual Basic (VB). VB itself is a development environment; the language element of that environment is VBA."
The minor differences
Hosted vs. stand-alone: In practical, terms, when most people say "VBA" they specifically mean "VBA when used in MS Office", and they say "VB6" to mean "VBA used in the last version of the standalone VBA compiler (i.e. Visual Studio 6)". The IDE and compiler bundled with MS Office is almost identical to Visual Studio 6, with the limitation that it does not allow compilation to stand-alone dll or exe files. This in turns means that classes defined in embedded VBA projects are not accessible from non-embedded COM consumers, because they cannot be registered.
Continued development: Microsoft stopped producing a stand-alone VBA compiler with Visual Studio 6, as they switched to the .NET runtime as the platform of choice. However, the MS Office team continues to maintain VBA, and even released a new version (VBA7) with a new VM (now just called VBA7.dll) starting with MS Office 2010. The only major difference is that VBA7 has both a 32- and 64-bit version and has a few enhancements to handle the differences between the two, specifically with regards to external API invocations.
VBA stands for Visual Basic for Applications and so is the small "for applications" scripting brother of VB. VBA is indeed available in Excel, but also in the other office applications.
With VB, one can create a stand-alone windows application, which is not possible with VBA.
It is possible for developers however to "embed" VBA in their own applications, as a scripting language to automate those applications.
Edit: From the VBA FAQ:
Q. What is Visual Basic for Applications?
A. Microsoft Visual Basic for Applications (VBA) is an embeddable programming environment designed to enable developers to build custom solutions using the full power of Microsoft Visual Basic. Developers using applications that host VBA can automate and extend the application functionality, shortening the development cycle of custom business solutions.
Note that VB.NET is even another language, which only shares syntax with VB.
It's VBA. VBA means Visual Basic for Applications, and it is used for macros on Office documents. It doesn't have access to VB.NET features, so it's more like a modified version of VB6, with add-ons to be able to work on the document (like Worksheet in VBA for Excel).
Source: Stackoverflow.com