The amount of alcohol in a standard drink varies by country, from 6 grams in Austria to 19.75 grams in Japan. (Japan, as a country, is in serious denial about the extent of their drinking problems. Binge drinking is a sort of national sport.)
In the United States, a standard drink — or unit — of alcohol is any drink that contains about 14 grams of pure alcohol (about 0.6 fluid ounces or 1.2 tablespoons). In the US, a standard drink is calculated as roughly the amount of alcohol that can be metabolized by the human body in one hour. In other words, if you drank one unit per hour you could (at least in theory) continue to drink indefinitely without becoming intoxicated. In fact, metabolic rates vary among individuals and by gender, so this is not really an accurate approximation. It does, however, give us something to work with.