The ALT tag, sometimes referred as the alternative tag, goes inside image codes to tell search engines what the image is about. An example of an ALT tag is:

<img src="moon.gif" alt="Picture of the Moon"/>

Many people forget to add the ALT tag to images, and some ignore it altogether. This is a mistake because the ALT tag is used by search engines not only to identify images but also for keyword density purposes.

The next time you place an image on your site do not forget to add the ALT tag with a couple of keywords pertinent to the image and to the content of the page.

Update: Equally important is the title tag inside the image code, some browsers (including some versions of Firefox) will not take into consideration the ALT tag, therefore having a title tag should make you more secure.