Attributes for <INPUT ...>
onFocus = "script command(s)"
onBlur = "script command(s)"

Usage Recommendation
use it, but don't rely on it

onFocus is the event handler for when the input field gets the focus. onBlur is the event handler for when the input field loses the focus. The "focus" indicates which object in the window reacts to keyboard input. If a text field has the focus, then if you type something in the keyboard, the letters appear in that field. Focus shifts from one object to another usually by clicking on them with the mouse or by hitting the tab key.

A common use for onFocus and onBlur is to put a prompt message in the status bar of the browser window. Before we show how this is done, we'd like to suggest that you not do this. Fooling around with the status bar is a classic way to annoy your readers. People tend to rely on the status bar to know what's going on, and it's irritating to find usual information gone.

That being said, in the interest of completeness here's an example of onFocus and onBlur used to change the status bar. Notice that we use onFocus to give the message we want, and onBlur to change back to the default.

<FORM ACTION="../cgi-bin/mycgi.pl" METHOD=POST>

name: <INPUT 
          NAME="realname" 
          onFocus = "window.status='Enter your name'" 
          onBlur  = "window.status=window.defaultStatus" 
          ><BR>
email: <INPUT 
          NAME="email" 
          onFocus = "window.status='Enter your email address'" 
          onBlur  = "window.status=window.defaultStatus" 
          >

<P><INPUT TYPE=SUBMIT VALUE="submit">
</FORM>

gives us

name:
email:

One final note about onFocus: don't use it to call an alert() box. Some browsers interpret the alert box as grabbing the focus, then when the box closed, the focus goes back to the input field, which again runs the onFocus event opening the alert box, etc. This endless loop is not a good way to impress your readers.