A power supply is an essential part of every electronic device. The whole system relies on it. If a power supply fails – the device dies too, and there is a chance that sensitive parts may be damaged. Power supply solutions vary from device to device. They may work using different techniques depending on needs. Tim has described probably the simplest power supply solution, which converts Alternating Current (AC) to Direct Current (DC). The supply circuit consists of well-known parts – transformer, which transforms voltage from 230V AC to 12VAC, follows diode bridge rectifier, smoothing using large caps and regulation with standard 7805 voltage regulator.
Such a solution proved itself in many designs. It is simple, cheap, and builds fast. Tim put everything in the short and informative post without overhead information. Following it, you will be able to construct a 5V DC power supply in an hour.