A machine designed for a purpose. Within the context of computer technology, a device is a unit of hardware, outside or inside the case or housing for the essential computer (the processor, memory, and data paths). A device is capable of providing input to the essential computer, receiving output, or both. When the term is used generally (as in, "computer devices"), it refers to keyboards, mice, monitors, hard drives, CD-ROM players, printers, speakers, microphones, and other hardware units.
Although a hard disk drive or CD-ROM drive may be physically inside the computer housing, they are considered devices because they are separately installable and replaceable. With notebooks and smaller computers, devices tend to be more physically integrated with the "non-device" part of the computer. The term "device" does not refer to the motherboard, the main processor, and the additional processors, (such as the numeric coprocessors) and random access memory (RAM).
The term "peripheral" (a truncation of "peripheral device") is sometimes used as a synonym for device or for any input/output unit. Some devices require special software (called device drivers), which you can downloadonline.