ABV is used to measure the alcohol content of beer, wine, distilled spirits, and other alcoholic beverages. Beers typically fall in the 3.0-13.0% ABV range, though some can be weaker or stronger than this.
IBU is an abbreviation for the International Bitterness Units scale, a gauge of beer's bitterness. What IBUs measure, are the parts per million of isohumulone found in a beer.
Isohumulone is the acid found in hops that give beer its bitter bite. Though the IBU scale can be used as a general guideline for taste, with lower IBUs corresponding to less bitterness and vice versa, it's important to note that malt and other flavors can mask the taste of bitterness in beer.