Alt Text Not Alt Tag
It's funny how many people call it "alt tag", when in reality, the alt attribute of the image, applet, and input tags is an attribute, not a tag. Most of the time, when someone is referring to the "alt tag" they are referring to the text within the tag - or the lack of that text. So I try to refer to it as "alt text". However, I'm sure if you do a search of this site you'll find places where I call it an alt tag, just like everyone else.
What is a Tag?
According to my glossary entry, a tag is "the markup characters that indicate the start or end of an element - but not the element content itself." This means that a tag in XHTML is the text that is surrounded by angle brackets, like:
<p> - a start tag </strong> - an end tag <img /> - a singleton tag (or singleton element)
What is an Attribute?
An attribute is "a part of an element that provides additional information about that element." So this would be any additional parts of a starting tag (or singleton) that provide more information about the tag:
<p style="color:red;"> <a href="index.htm"> <img src="image.gif" />
Alt Text is the Contents of the Alt Attribute
Once you've got an alt attribute on your element, then the text within that attribute is the alt text. This is the alternative text that people who are visiting Web sites with images off, with screen readers or automated tools like search engine spiders that is displayed and read.
Alt Text is Not a Tool Tip
Only Internet Explorer supports displaying alt text as a tool tip. And this may change. It's a good idea to use the title attribute when you need a tool tip rather than relying on the alt attribute which was not intended for that use. In fact, you should be using the title attribute for many of the things that people use the alt attribute for.