A DLL can contain everything that programs might require to run properly, such as functions, graphics or data lists. If a DLL has been by mistake deleted, corrupted or just not installed, it can reason a lot of different types of runtime errors.
Only a Resource
DLLs may include small programs, but they do not run by themselves. DLLs are just repositories of normally used resources that other programs use.
Many programs use identical parts of code or other data. Before making larger programs by including these parts in every program, the common pieces are put in a DLL where all the programs can use the same copy.
Not only do DLLs save disk storage space in the computer, but they do not have to be loaded into random access memory (RAM) in order to be used. This makes the system run more efficiently and leaves more memory in which applications can run.
DLLs are particularly useful when designing OS and other large programs. Sinking the amount of code in the system makes it much easier to find problems when they exist; there is only one copy of the code to check for errors, rather than 10, 20 or 100 copies.
The DLL concept is actually used in a different of ways; not all are called DLLs. E.g, a device driver is a form of DLL, as is an ActiveX control.