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Under Windows, DLL (Dynamically-Linked Libraries) are generally used to improve code modularity and sharing. A DLL can be loaded automatically when the program is loaded (if it requires the DLL). The program can also explicitly request Windows to load a DLL at any moment during runtime, using the LoadLibrary function from the Win32 API.

This naturally suggests to use DLLs as a plugin mechanism. For instance, a web server could load extensions modules stored in DLLs at runtime. But Windows does not really make it easy to implement plugins that way. The reason is that when you try to create a DLL from a set of object files, the linker needs to resolve all the symbols, which leads to the very problem solved by

FlexDLL:
Windows DLL cannot refer to symbols defined in the main application or in previously loaded DLLs.
Some usual solutions exist, but they are not very flexible. A notable exception is the edll library (its homepage also describes the usual solutions), which follows a rather drastic approach; indeed, edll implements a new dynamic linker which can directly load object files (without creating a Windows DLL).
FlexDLL is another solution to the same problem. Contrary to edll, it relies on the native static and dynamic linkers. Also, it works both with the Microsoft environment (MS linker, Visual Studio compilers) and with Cygwin (GNU linker and compilers, in Cygwin or MinGW mode). Actually, FlexDLL implements mostly the usual dlopen POSIX API, without trying to be fully conformant though (e.g. it does not respect the official priority ordering for symbol resolution). This should make it easy to port applications developped for Unix.