Definition: Analogue (analog) Electronics
The main objective of any electronic circuit is to transfer and manipulate information. This information is represented as either an electrical voltage or a current, but mostly voltage. In an analogue system the voltage itself carries the value of the information it is supposed to transmit.
For example, if a pressure sensor is measuring a value of 1kPA and it has to encode that value as an electrical signal in an analogue system, it could send a voltage of 1V. For 6kPA it would send a voltage of 6V and so on. Thus, the voltage value that carries the information can be any value between a minimum and maximum. Although this type of information encoding is very simple and easy, the main disadvantage is that it is susceptible to noise. For example, if a noise spike of 1V is generated by some other component that sits next to the pressure sensor it will change the voltage from 3V to 4V. This will incorrectly make it seem like the pressure is 4kPA and not 3kPA.
Also see: Digital Electronics