The operation of electronic devices depends on a fundamental property of matter called electric charge. There are two kinds of electric charge: positive and negative. Atoms contain both positively charged particles (protons) and negatively charged particles (electrons). In electrical devices an electromotive force, or voltage, is created that causes electrically charged particles to move. The movement of charged particles is called an electric current. In electronic devices and in most other electrical devices, the current consists of a flow of electrons; in other electrical devices, such as batteries, the current consists of a flow of ions (atoms lacking or having an excess of electrons).

In most nonelectronic devices, electricity supplies energy for such purposes as heating or lighting, or running electric motors. In most electronic devices, electricity is used to represent and convey some type of information. The information may be simple, such as an indication that the door of an automobile is open, or it may be complex, such as the sounds produced by a group of musical instruments. The changes in voltage or current that are used to represent information are called electrical signals.

An analog electronic device works with continuously varying electrical signals. These signals are typically used to represent quantities that vary continuously. For example, the radio waves used in AM radio have continuously varying amplitudes that represent the varying pitch and loudness of a sound, and television signals have continuously varying frequencies to represent the varying patterns of brightness and darkness of a scene.

A digital electronic device works with sequences of pulse-like signals. These signals are coded to represent numbers, making them especially useful for operations that require numerical calculations, as in a digital computer. In many digital electronic devices, a series of numerical values can be used to represent a continuously varying quantity. For example, the sound recorded on an audio compact disc is recorded digitally.

The representation of the way an electric current or voltage varies with time is called a waveform. Analog signals generally have smoothly varying waveforms called sine waves. Digital signals generally have rectangular waveforms, rising and falling abruptly between two levels.

In general, electronic systems are designed to detect various kinds of signals. They then modify these signals or produce new signals based on the signals they detect. For example, a compact disc player detects the variations in the light reflected from the surface of a rotating disc. Using this information, it produces an electric current that re-creates the sound by means of earphones or loudspeakers.

One of the most important functions performed by electronic systems is the strengthening, or amplification, of weak signals. In a sound system, for example, a compact disc player produces only a weak signal; through amplification, the system can produce the much stronger signals required by earphones or loudspeakers.

Other important functions carried out by electronic systems include rectification, the conversion of an alternating current to a direct current; oscillation, the production of various kinds of regular waveforms from a direct current; and switching operations called logic functions.