In answer to the above replies. exe can’t  be upgraded you have to replace the exe with a new one. While dll can be upgraded without replacing the entire project.

Ya,Anju the main difference is here-

An exe is an executible program. A DLL (Dynamic Link Library) is a file that can be loaded and executed by programs dynamically. Basically it's an external code repository for programs. Since usually several different programs reuse the same DLL instead of having that code in their own file, this dramatically reduces required storage space. A synonym for a DLL would be library.

DLL does not have main function but exe has main function

Here DLL is inprocess component, both component and consumer will share same memory and Exe is out process component, it will run in its own memory.