IBU is a number often listed on the side of a beer. It stands for International Bitterness Units. This is the measure of how much bitter hop resin (Alpha Acid) is in the beer. The old rule of thumb is that a higher IBU means a hoppy beer and a low IBU equates to a sweet beer. However, these days too much emphasis is placed on IBUs by undereducated beer geeks. IBU alone will not inform the consumer how bitter or how hoppy a given beer will taste. In fact, IBU alone will tell the consumer nothing about the beer except for how much Alpha Acid is in the brew. For example, most consider Barely Wine to be an extremely sweet style of beer with little to no hop flavor. A typical Barley Wine contains more than 75 IBU. Now, compare that with a standard pale ale like Sierra Nevada Pale Ale. Many consider this beer to be slightly bitter with a good hop flavor. The Sierra Nevada Pale ale has HALF the IBU rating, weighing in at 35 IBU. A side by comparison with Guinness, which is considered smooth and sweet with little to no hop flavor, show that Guiness has 45+IBU. That is 15 IBU higher than Sierra Nevada's Pale Ale. Budweiser's rating is a measly 18 IBU. The moral or message to take away from this is do not be fooled by people who over emphasis the importance of a beer's IBU to the beer's quality or taste.
What should you take away from the IBU value? Well, if you new to the beer world use the measure with a grain of salt. Like sweeter beers? Then shy away from high IBU numbers at first. Like a hoppy beer by all means aim for a higher IBU rated can on the shelf. However, do not be surprised if that 20IBU ale you ordered is too bitter to finish and that 100IBU ale you buddy ordered tastes strongly of sugar and molasses without any hop flavor.
Read up on BU:GU for another way to gauge the bitterness and sweetness to a beer.