This article is from the Beer and Other Brewing FAQ, by Kurt Swanson email@example.com with numerous contributions by others.
Alpha acids are bittering compounds found in hops that are extracted
when hops are boiled with wort. The alpha acid "rating" on hops
describes how much of the weight of the hop is made up of alpha acids.
Hops with a higher alpha acid content will contribute more bitterness
than a low alpha hop when using the same amount of hop.
HBU stands for "Homebrew Bitterness Unit", which is a recipe unit
for hops. It takes into account the alpha acid content of the hop,
so that a recipe will call for a certain amount of HBU's rather than
an amount specified in ounces. HBU is computed by multiplying the
weight of hops in oz. by the alpha acid percentage of the hops; sum
for all hop additions. For example, 1 oz of 7% alpha hops will have
a HBU of 7. Note that volume is ignored in the HBU, therefore it
is important to include the volume of the recipe, or express the
hop additions in HBU per gallon (or HBU per 5 gallons) rather than
just strictly HBU.
IBU stands for "International Bittering Unit", and is a measure of
the amount of bittering compounds in a particular volume of beer,
rather than a recipe unit. However, the "Hops and Beer" special
issue of Zymurgy (see Bibliography) presents a formula for estimating
IBU, considering several variables -- alpha acid content, wort volume,
wort gravity, and time in the boil.
Another way to think of this is that HBU represents the "potential"
for bittering beer (the bittering strength of the hops), while IBU
represents "actual" bittering, and is a measure of the beer, not