Hydrometers and refractometer are used to measure the amount of sugar in your wort. The typical beer hydrometer can also predict potential alcohol in the finished beer by taking a specific gravity at the beginning and end of fermentation. The hydrometer can also be used to determine when fermentation is complete.
The refractometer is a great tool for measuring sugar content in the kettle and allow for adjustment when necessary. The beer refractometer requires only a few drops of wort to take a reading and many have automatic temperature compensation adjustment built in. If you want to use your hydrometer after fermentation you will need a refractometer calculator to get accurate results as alcohol distorts the final reading.