The <acronym> tag is similar to the <abbr> tag but is used with acronym. The difference between the two is that acronyms are usually the first letter of every words in a multi-words expression like: FBI, USA, UN, etc. Abbreviation is a shorter way of writing a word like: abbr.,cpt.,sgt.,etc. Some browser will show the full word in a bubble when the viewer hovers his mouse over the word.

Ex: