What is the Full Form of CD ?

CD - Compact Disc

Compact Disc is an optical disc used to store data including video, text, audio etc. in digital form. The Data is written to the CD using an optical device known as CD Writer which can be read through a similar device known as CD Drive.

Size & Capacity
Standard CDs have a diameter of 4.7 inch, 1.2 millimeters thick, made up of polycarbonate plastic and can hold data up 700 MBs. Compact Disc are far cheaper than HDD and easy to handle.

CD - Change Directory

Change Directory is a Disk Operating System (DOS) command used to change the directory (also known as a folder in a Computer System).   

For ex. If you want to enter in to the directory "Playlist" form d: drive, the change directory command for the same will be :-

d:\> cd Playlist