A hydrometer is a device used to measure the specific gravity of unfermented wort and fermented beer during the brewing process. The most common form of brewing hydrometer is a hollow glass device with a metal weight at the bottom. A paper scale is included inside the tube, and usually marked in one or more scales. The hydrometer floats freely in the liquid you are measuring, and is read at the point where the surface of the liquid touches the stem. The most frequently used scale for brewing is the specific gravity scale. The specific gravity scale provides a relative measure of sugar as compared to water, and is typically expressed to three digits after the decimal point as in 1.040.
Using a Hydrometer
The proper way to use a hydrometer is to draw a small quantity of wort or beer into a sampling tube or test jar, and then insert the hydrometer into the sample. Do not use the plastic tube the hydrometer came in for measuring a sample as it is too small for a proper measurement. Take your reading on the appropriate scale by reading where the surface of the sample crosses the scale on the hydrometer.
Adjusting for Temperature
The most common type of hydrometer sold for brewing is calibrated for use on water at temperature of 60 F. If the liquid you are measuring is at a different temperature, you need to make an adjustment to account for the difference between calibration temperature and the liquid's temperature. You can perform this calculation with a brewing calculator such as BeerSmith or you can use a hydrometer adjustment table such as this one.