DLL way

Ive read the article about "How DLL's tick" on MSDN, and I read a couple of tutorials about DLL's, and DLL injection, but I still dont seem to understand a couple of things.

First of all, most tutorials speak of intercepting a function within DLL injection.I don't really understand, what they mean by that.Do you mean like, alter a function, so it fits your desires?Or you can only add some code to the real function?

Second, what can you actually do with an DLL?For example, is it possible to write a log from everything that happens for example within mIRC?Is it possible, to do whatever the main program does?

Third, some programs give information about API's, and such.Can you use them in DLL's?

Fourth, I've been testing DLL's in mIRC and in another program which supports custom DLL's.I started with a simple Messagebox.In both cases, it has loaded the Messagebox, and shows it on screen, but as soon as I click Ok, the program crashes, for some reason?

Thanks

You read a poorly written article. I know it. Try these instead of this stupid one I suppose you read

http://www.codeproject.com/system/hooksys.asp
http://www.codeguru.com/cpp/w-p/syst...cle.php/c5767/

as for DLL API:
http://msdn.microsoft.com/library/de..._libraries.asp

as for injection:
http://www.codeproject.com/threads/winspy.asp