Racking is the process of transferring beer from one brewing vessel to another. Beer is typically racked utilizing a racking arm, racking cane, or racking tube. The beer is either pumped from one vessel to another, or a siphon or CO2 tank is used to create a positive or negative pressure and gravity is used to rack the beer.
A wide selection of home brewing beer racking accessories can be found here:
Beer being racked from a primary fermenter to a secondary fermenter using a racking cane and tube: