What is it?
A hexadecimal (hex) editor (also called a binary file editor or byte editor) is a computer program you can use to manipulate the fundamental binary data that constitutes a computer file. The name “hex” comes from “hexadecimal,” a standard numerical format for representing binary data. A typical computer file occupies multiple areas on the platter(s) of a disk drive, whose contents are combined to form the file. Hex editors that are designed to parse and edit sector data from the physical segments of floppy or hard disks are sometimes called sector editors or disk editors.
What is it used for?
A hex editor is used to see or edit the raw, exact contents of a file. Hex editors may used to correct data corrupted by a system or application.
Where can I get one?
A list of editors can be found on the forensics Wiki. You can download one and install it on your system.