A water tap is simply a valve regulating the flow of water to an outlet. Taps can deliver either hot or cold separately, or mixed (often known as a flick mixer). A separate hot and/or cold tap is plumbed directly to the supply pipes and usually requires the whole system to be shut down and drained in order to complete their servicing. A mixer tap (flick mixer) should be connected to individual isolation valves beneath it to allow for servicing of the tap without disrupting the rest of the supply.
Separate taps: It is common for washers to simply wear out. When servicing an individual tap the handle and spindle must be removed to access the tap washer. They were often made of leather but now can be made of rubber and/or plastic combinations. Sometimes replacing the washer may not stop a leak. There are also o rings installed onto the thread of the spindle (the part that holds the tap handle) or a body washer that may be faulty. In some cases the seat of the tap body is worn and wont seal without being ground back to flat.
Mixer taps: The most common faults with these taps is that they leak from their bodies or stems, or they wont shut off. Once isolated, the mixer tap can be dismantled by removing the tap handle. This exposes the cartridge, which is where the hot and cold water gets mixed. A mixer cartridge is replaceable and can wear out or fail due to excess pressure. Most mixer taps are only warrantied to 500 KPA pressure. It is recommended that when changing or repairing a mixer tap the pressure is tested and checked against manufacturers warranty.
It is not essential to employ a licensed plumber to repair a tap, however it is often cheaper and simpler, as they have all the tools and knowledge needed to complete a repair.