Hexadecimal (HEX) Editor
Most of the editors we use like notepad or wordpad are called as text editors
A plain text file is represented and edited by showing all the characters as they are present in the file.
They read the file and show the content in it in ASCII character (in most cases).
In some cases we need hexadecimal editors (binary file editor or byte editor).
For example if you try to open a .gif file which is a binary file in notepad, you will see junk characters as because
notepad tries to convert binary code to ASCII code any tries to display them as characters.
A good hex editors for windows is HxD
There are others like 010 Editor, BIEW, bvi, Emacs.
Normal text editors wont be helpful when you are trying to see or analyze the code of images or others binary files.