The Dean-Stark apparatus is designed to collect water produced in synthetic reactions carried out under reflux. The reactions are normally done in solvents (for example toluene) that remove water formed in a reaction as an azeotrope. The condensed mixture of toluene and water collects in the burette and the denser water separates and falls to the bottom. The tap allows the water to be removed.
the Dean-Stark apparatus in the laboratory typically consists of vertical cylindrical piece of glass, often with a volumetric graduation on its full length and a precision tap on the bottom very much like aburette. The top of the cylinder is a fit with the bottom of the reflux condenser. Protruding from the top the cylinder has a side-arm sloping toward the reaction flask. At the end the side-arm makes a sharp turn so that the end of the side arm is vertical as well. This end connects with the reactor.
During the reaction in, vapors containing the reaction solvent and the component to be removed travel out of reaction flask up into the condenser, and then drip into the distilling trap. Here, immiscible liquids separate into layers. When the top (less dense) layer reaches the level of the side-arm it can flow back to the reactor, while the bottom layer remains in the trap. The trap is at full capacity when the lower level reaches the level of the side-arm--beyond this point, the lower layer would start to flow back into the reactor as well. It is therefore important to syphon or drain the lower layer from the Dean-Stark apparatus as much as needed.
the image of this apparatus can be seen on the link provided