In brewing, specific gravity is defined as the ratio of the density of a brewing liquid, such as wort or beer, as compared to the density of pure water. Typically either a hydrometer or a refractometer is used to determine the specific gravity of beer or wort.
A wide selection of home brewing hydrometers and refractometers can be found here:
Hydrometers and Refractometers
Below is a photo of a hydrometer, one of the instruments used to take a specific gravity reading in brewing.