Tags are <body>, <a>, etc. They're also known as "HTML elements" within the DOM. Attributes are things like title=, alt=, etc. Properties are parts of CSS, for instance background-color in <body style="background-color: red; font-weight: bold;">. Those are the 3 things people get mixed up most often.
alt is a required attribute. Title isn't required, but I recommend including title. Title should be just a single word or phrase, whereas alt should be a sentence or two, describing what a missing image should result in, or a text-only browser or browser with images turned off should see. For some unimportant images you could put in alt="", or navigation images, something like alt="[Downloads]". In the case of navigation images, you could make the title title="click here to download", since even though there is no rule on this in the specification, browsers generally honor title over alt when showing a tooltip/titletip (text that appears in a yellow box when you hover over an image).
