One US customary cup holds 8 US fluid ounces, which is half a US pint, or about 237 milliliters. Fluid ounces measure volume, so this does not mean a cup of a dry ingredient weighs 8 ounces.
Measuring cups sold for cooking are often labeled as 240 milliliters in the US, while the metric cup used in countries such as Australia and Canada is 250 milliliters.
Source: Wikipedia, Cup (unit)