Definitionally they seem the same, yet they are deemed dissimilar enough by the W3C to have separate tag definitions. What is the semantic difference between <acronym> and <abbr> in terms of linguistics, the HTML spec and interpretation.
An acronym is made of the initial letters of some words (e.g.Hypertext Mark-up Language => HTML), while an abbreviation is a shortened form of a word (e.g. abbreviation => abbr).
Wikipedia gives a more accurate view of the nitty-gritty details.