IO, or Input/Output, devices refer to any physical devices that interact with a CPU. Input devices send signals to a computer and output devices allow for computers to send information out from a computer.
When it comes to devices, we mainly think of examples that require direct human interaction; however, the large range of IO devices can be categorized into three categories: human-readable, machine-readable, and communication.
Human readable devices are devices that can be interpreted/understood in a natural language structure by humans. Some examples include printers, keyboards, and a mouse.
Machine readable devices are devices that are formatted to allow communication between different hardware, without the need for human interpretation. Some examples include hard drives/disks, controllers, and SD cards.
Communication devices are devices that allow devices to interact over a network. A network is a set of devices that are linked to share some resources over a shared medium. Some examples of communication devices include modems and Bluetooth adapters.
Take a look at the table to the right - it organizes some IO hardware devices into the three categories discussed above.