The magic variables %n contains the arguments used to invoke the file: %0 is the path to the bat-file itself, %1 is the first argument after, %2 is the second and so on.
Since the arguments are often file paths, there is some additional syntax to extract parts of the path. ~d is drive, ~p is the path (without drive), ~n is the file name. They can be combined so ~dp is drive+path.
%~dp0 is therefore pretty useful in a bat: it is the folder in which the executing bat file resides.
You can also get other kinds of meta info about the file: ~t is the timestamp, ~z is the size.
Look here for a reference for all command line commands. The tilde-magic codes are described under for.
What is a DLL?
Dynamic Link Libraries (DLL)s are like EXEs but they are not directly executable. They are similar to .so files in Linux/Unix. That is to say, DLLs are MS's implementation of shared libraries.
DLLs are so much like an EXE that the file format itself is the same. Both EXE and DLLs are based on the Portable Executable (PE) file format. DLLs can also contain COM components and .NET libraries.
What does a DLL contain?
A DLL contains functions, classes, variables, UIs and resources (such as icons, images, files, ...) that an EXE, or other DLL uses.
Types of libraries:
On virtually all operating systems, there are 2 types of libraries. Static libraries and dynamic libraries. In windows the file extensions are as follows: Static libraries (.lib) and dynamic libraries (.dll). The main difference is that static libraries are linked to the executable at compile time; whereas dynamic linked libraries are not linked until run-time.
More on static and dynamic libraries:
You don't normally see static libraries though on your computer, because a static library is embedded directly inside of a module (EXE or DLL). A dynamic library is a stand-alone file.
A DLL can be changed at any time and is only loaded at runtime when an EXE explicitly loads the DLL. A static library cannot be changed once it is compiled within the EXE.
A DLL can be updated individually without updating the EXE itself.
Loading a DLL:
A program loads a DLL at startup, via the Win32 API LoadLibrary, or when it is a dependency of another DLL. A program uses the GetProcAddress to load a function or LoadResource to load a resource.
Further reading:
Please check MSDN or Wikipedia for further reading. Also the sources of this answer.
Best Solution
Directly from the Windows.h header file:
If you want to know what each of the headers actually do, typing the header names into the search in the MSDN library will usually produce a list of the functions in that header file.
Also, from Microsoft's support page:
Finally, if you choose to use either of these preprocessor defines, and something you need is missing, you can just include that specific header file yourself. Typing the name of the function you're after into MSDN will usually produce an entry which will tell you which header to include if you want to use it, at the bottom of the page.