If the DLL is from a third-party vendor you may have some luck if you Google it. If it is a COM DLL then you can use the Object Browser in Visual Studio, you will get the method/fuction signature but not the purpose.
Come to think of it, you can use the Dependency Walker tool, which is part of Visual Studio 6. You can also get it here: http://www.dependencywalker.com/
Hope this helps.