Many users are used to editors that are normally in overwrite mode: when you backspace and start typing, you "type over" the character that is underneath the cursor.  By default, Emacs works in insertion mode, where new characters are inserted just before the cursor's position.
 This includes some mainframe editors, like XEDIT, and (in my memory) a lot of older tools for word processing and general editing.
If you prefer overwrite mode, just give the command
overwrite-mode. You can use
to shorten this to
ov. On many
keyboards, pressing INSERT also turns on overwrite mode. If you
get tired of overwrite mode, use the same command to turn it off.
If you always want to use overwrite mode, create a file named .emacs in your home directory, and put the following line in it:
(setq-default overwrite-mode t)
This is a simple Emacs customization; for a lot more about customization, see O'Reilly & Associates' Learning GNU Emacs, by Debra Cameron and Bill Rosenblatt.