The device is used to change gear; in a manual transmission vehicle this will normally be done whilst depressing the clutch pedal with the left foot to disengage the engine from the drivetrain and wheels.
The gear knob (or shifter knob) is at the end of the gear stick; it forms the handle of the gear stick. Typically the gear knob includes a diagram of the shift pattern of the gear selection system; i.e. the positions to which the gear stick should be moved when selecting a gear.
A typical manual transmission vehicle, with five forward gears, will have seven possible positions: the five forwards gears, reverse gear, and a central "neutral" position. Some vehicles have a special button to prevent accidental engagement of reverse. Others require that the lever be lifted, pressed down, or moved with extra force to engage reverse.
In some rare cases, the gear stick and gear box in general can completely shift, re-aligning where the gears are located. This places 1st gear where 3rd usually is, and 2nd, 3rd and 4th where 4th, 5th and reverse normally are respectively. This leaves no room for 5th gear or reverse, subsequently making the car unable to use these gears at all. Mitsubishi Magna TR's are most known for doing this, and also usually won't go into 3rd gear either.
Many automatic transmission vehicles have extra controls on the gear stick, or very close by, which modify the choices made by the transmission system depending on engine and road speed; for example, "sports" or "economy" modes which will broadly speaking allow, respectively, for higher and lower revolutions per gear, before changing up.
Some specialist vehicles have controls for other functions on the gear stick. The Land Rover Freelander introduced a button for that company's Hill Descent Control system feature, which uses the brakes to simulate the function of a low-ratio gearbox in steep descents.