The <abbr> and <acronym> tags allow you to give information about the meaning of an abbreviation or acronym used in your text without disrupting the flow of text by placing the information directly in the text. This is done by putting the information in the tag's title attribute. The browser will then display the information as a tooltip.
The first thing you might ask is; what's the difference between an abbreviation and an acronym? Well, they're both a group of letters used as a shortened forms of a word, phrase or name. Whereas an abbreviation can be anything you desire, its purpose being to replace the word or phrase in the text, an acronym is usually a standard group of letters used by everyone and that standard group of letters is usually articulated as a standard word.
Here's an example use of the <abbr> tag:
In the csmdb you can configure your Start menu.
Here's an example use of the <acronym> tag:
Because of its age, NASA has decided to retire the Space Shuttle.
By the way, I know the above sentence is bad grammar, but I wanted to put the acronym in the middle of the sentence. Also, you don't need the <abbr> or <acronym> tags to create a tooltip, you can add the title attribute to span (or almost any other html element) and you'll get a tooltip, but only the <abbr> or <acronym> tags will display the dotted underline to indicate that a tooltip is available.
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