An SLL is a Static Link Library. It consists of a set of Classes, Subs, and Functions which are compiled into a machine-code library. Since it is a library, the code cannot be executed standalone. It functions much like a DLL would, but the pre-compiled machine code is actually embedded into the final .EXE or .DLL to reduce the number of files in your project.
A Static Link Library is the perfect vehicle for third-party code, because it creates a single final module while not requiring source code to be distributed. It allows you to create a group of your own libraries, which you know function correctly and don't require any further debugging. It also offers big advantages to larger group programming projects to control distribution of various elements.
See Also
Creating a Static Link Library