Electronics

Electronics is the study of electricity (the flow of electrons) and how to use that to build things like computers. It uses circuits that are made with parts called components and connecting wires to do useful things. The science behind Electronics comes from the study of physics and gets applied in real-life ways through the field of electrical engineering.
Many people can name several simple electronic components, such as transistors, fuses, circuit breakers, batteries, motors, transformers, LEDs and bulbs, but as the number of components starts to increase, it often helps to think in terms of smaller systems or blocks, which can be connected together to do something useful.
One way of looking at an electronic system is to separate it into three parts:
Inputs - Electrical or mechanical sensors, which take signals from the physical world (in the form of temperature, pressure, etc.) and convert them into electric current and voltage signals.
Signal processing circuits - These consist of electronic components connected together to manipulate, interpret and transform the information contained in the signals.
Outputs - Actuators or other devices that transform current and voltage signals back into human readable information.
A television set, for example, has as its input a broadcast signal received from an antenna, or for cable television, a cable.
Signal processing circuits inside the television set use the brightness, colour, and sound information contained in the received signal to control the television set's output devices. The display output device may be a cathode ray tube (CRT) or a plasma or liquid crystal display screen. The audio output device might be a magnetically driven audio speaker. The display output devices convert the signal processing circuits' brightness and colour information into the visible image displayed on a screen. The audio output device converts the processed sound information into sounds that can be heard by listeners.
Analysis of a circuit/network involves knowing the input and the signal processing circuit, and finding out the output. Knowing the input and output and finding out or designing the signal processing part is called synthesis.
History
People started experimenting with electricity as early as 600 B.C.E, when Thales of Miletus discovered rubbing fur on amber would cause them to attract each other.
Starting in the 1900s, devices used glass or metal vacuum tubes to control the flow of electricity. With these components a low power voltage can be used to change another. This revolutionized radio, and allowed other inventions.
In the 1960s and early 1970s transistors and semiconductor began replacing vacuum tubes. Transistors can be made much smaller than vacuum tubes and they can work using less energy.
At about the same time, integrated circuits (circuits that are integrated inside other circuits) became commonly used. Integrated circuits made it possible to reduce the number of parts needed to make electronic products and made the products much cheaper in general.
Analog circuits
Analog circuits are used for signals that have a range of amplitudes. In general, analog circuits measure or control the amplitude of signals. In the early days of electronics, all electronic devices used analog circuits. The frequency of the analog circuit is often measured or controlled in analog signal processing. Even though more digital circuits are now made, analog circuits will always be necessary since the world and its people work in analog ways.
Radar
Pulse circuits are used for signals that require rapid pulses of energy. For example, a radar works by using pulse circuits to create and send high powered bursts of radio energy from radar transmitters. Radar antennas are used to send ("transmit") the high powered bursts in the direction the antenna is pointed.
The radar transmitter's pulses or bursts of radio energy hit and bounce back (they are "reflected") from hard or big metallic objects. Hard objects are things like buildings, hills, and mountains. Big things made of metal include aircraft, bridges, or even objects in space, like satellites. The reflected radar energy is detected by radar pulse receivers which use both pulse and digital circuits together. The pulse and digital circuits in radar pulse receivers are used to show the location, distance, and speed of objects which have reflected the radar transmitter's high powered pulses.
Digital circuits
Digital circuits are used for signals that only turn on and off instead of often working at levels somewhere between on and off. Active components in digital circuits typically have one signal level when turned on, and another signal level when turned off. In general, in digital circuits a component is only switched on and off.
Computers and electronic clocks are examples of electronic devices that are made up of mostly digital circuits.