Usually, alcohol is measured by percent of volume (abv). In America, the alcohol content in beer is measured by weight (abw). Because alcohol weighs about 20% less compared to water, abw measures seem 20% less than the abv measures for the similar alcohol amount. In Europe, the strength of beer is measured by the fermentables in the wort.
But just recently, Britain made use of OG (original gravity) -- 1000 times the ratio of the wort gravity to that of water. So a beer that has an OG of 1040 is 4% denser compared to water, due to dissolved sugars. Generally, you can take 1/10 of the last 2 digits to approximate the alcohol percentage by volume when dissolved sugars become fermented. In this example, the abv will be around 4% (40/10 = 4%) Right now, British beer is taxed based on the actual % ABV instead of the older OG so you can find both of them displayed.