There are two types of chlorine: DICHLOR and TRICHLOR. There are different uses for both chlorines.
Dichlor is the recommended type of chlorine to use in the hot tub. This type of chlorine usually comes in the form of a granular powder and not in a tablet form. It quickly dissolves to prevent any corrosion to the equipment or damage to the shell of the hot tub.
Trichlor is almost always in the form of a tablet. This type of chlorine is best for swimming pools. Trichlor tablets can dispense too much chlorine at once, making it extremely corrosive to your hot tub equipment (pumps, seals, hot tub shell, and heater).
It is extremely important to use quality chemicals to preserve your investment; your hot tub. Chemicals that are cheaply manufactured can be very harmful to your hot tub and are less potent, which will actually cost you much more in the long run.