:: A Dynamic Link library, or DLL, is a collection of routines that can be called by applications and by other DLLs. Like units, DLLs contain sharable code or resources, they provide the ability for multiple applications to share a single copy of a routine they have in common.
Here are some DLL related tutorials and articles for you:
Dynamic Link Libraries and Delphi - Everything you ever wanted to know about DLLs and Delphi but didn't know where to look for answers (or were to afraid to ask). Papers, tutorials, articles, code, what, how, why, when, where...
Controling DLL loading and initialization - When you need to control the loading and unloading of a DLL, or if you want to initialize your DLL when it is loaded, you can have a function called DllMain do your initialization...
Delphi memory manager problems in dynamic libraries - An info on Delphi memory allocation and dynamic-link libraries (DLL). Why and when an "Invalid pointer operation" exception strikes and how to fight against it. How to properly use strings as parameters in DLL functions.
DLLs Made Simpler - This article is aimed at removing the fear of writing DLLs for beginners. A lot of beginners think DLLs are really complex, when actually they are pretty simple.
DLL inside a Delphi EXE - Here's how to store a DLL inside a Delphi program exe file as a resource.
BPL vs. DLL - An overview of creating and using 'Borland package libraries' and how they compare with 'dynamic link libraries' in creating smaller Delphi applications.
Dynamic World of Packages - See how to store a Delphi form inside a BPL. Find out about dynamically embedding forms into an application - building applications your users can modify at runtime.
Registering DLL and ActiveX controls from code - How to register (and unregister) OLE controls such as dynamic-link library (DLL) or ActiveX Controls (OCX) files from a Delphi application.