Putting it simply, it is the act of entering data into a computer. Once the data has been entered into the computer it can be processed and the inputted instruction can be carried out.
In addition to coming from an external device, an input can come from a piece of software. This data is typically dealt with automatically.
What input devices are there?
There is a massive range of objects that can serve as input devices. These can include the most simple ones, such as a keyboard, mouse or touchscreen, as well as less obvious examples like graphics tablets or purpose-built controllers designed to interface with software such as digital musical instruments.
In- vs Output
Input/Output is the communication between an information processing system, such as a computer, and the outside world. Inputs are the signals or data received by the system and outputs are the signals or data sent from it.
An I/O interface is required whenever the I/O device is driven by the processor and the interface must have the necessary logic to interpret the device address generated by the processor.
Outputs may include text, audio or video and will typically come from an output device such as a speaker or screen. They can also come in the form of raw data, which can be interpreted by a reader or used as input for another piece of software.