title tags and alt tags are both major for accessibility...
alt tags are 'alternative' for images, they tell a speech browser what to say for example "Web Pro World Logo" or "top right corner" they describe the image.
title attributes are to be displayed in the mouse tool tip (yes they override IE's display of the alt attribute) and they provide additional information to the site visitor, example: "view the home page" "Contact Web Pro World" on images they are not always required, and tend to be more advantageous when used with links <a>.
Images only (not links) you might have alt="company employees" title="from left to right: names of employees in photo"... on mouse over the title will show, in a speech browser or text only browser the user would see company employees.
both alt and title can be picked up in search engines. Do not use either to stuff your site with keywords..
hope that helps!