A peripheral device, more commonly referred to as a peripheral, is a "device attached to a host computer behind the chipset whose primary functionality is dependent upon the host...".
Thus, a peripheral is any device you attach to your computer which is not central to your computer's standard operation. Printers, scanners, game controllers, and security dongles are all examples of peripherals.
This definition can be found here: http://en.wikipedia.org/wiki/Peripherals
The term "hardware device," however, seems a redundant phrasing, since any "device" by definition would be hardware.
The best example of a central device is the computer. these days a number of peripheral devices can be attached such as printers, scanners, routers, Uninterpretable Power Supplies, a long list of storage devices
Printers, scanners, game controllers, and security dongles are all examples of peripherals.
A modem is a peripheral device based on the fact that peripheral devices are devices located on the outside of a computer