What are file extensions ?

Question

such as .dll .exe. .ini etc etc

Answer 1

Exactly as danny said. They tell your computer what type of file it is and therefore what to use to open the file.

.exe, for example, stands for Executable. It's basically a program that doesn't depend on other programs to open it.

.psd is a Photoshop file. It depends on Photoshop to open it.

The file extension tells the computer this, and the first time you open a file with a new extension, the computer saves this to memory and associates all extensions with that program.

Answer 2

they tell the OS what kind of file it is.