A hydrometer is an instrument that measures the specific gravity or relative density of a liquid in relation to water. They are typically constructed of a glass cylinder (similar to laboratory cylinders), complete with a weighted bulb that floats in liquid. Hydrometers are divided into two categories, those liquids that are heavier than water and those that are lighter than water.
To operate the hydrometer, you place it in the liquid and take a reading from where the hydrometer breaches the surface. This reading will give you the specific gravity of the water. In high density liquids the hydrometer will float much higher than in low density liquids. Hydrometers can also be used to measure the temperature at a specific range depending upon your specific requirements.
Specialised variants are available depending on their application. For example, a lactometer is used to check the purity of dairy products, whereas an alcoholmeter indicates the alcoholic strength of a liquid (where water and alcohol have been mixed) and a brix hydrometer is used to measure the percentage of sugar and is often used within wine production.