Encoding:

Encoding is the process of putting a sequence of characters into a special format for transmission or storage.

Decoding:

To convert from a coded form into the original form.

Transcoding:

The process of converting a media file or object from one format to another. Transcoding is often used to convert video formats (i.e., Beta to VHS, VHS to QuickTime, QuickTime to MPEG). But it is also used to fit HTML files and graphics files to the unique constraints of mobile devices and other Web-enabled products. These devices usually have smaller screen sizes, lower memory, and slower bandwidth rates. In this scenario, transcoding is performed by a transcoding proxy server or device, which receives the requested document or file and uses a specified annotation to adapt it to the client.