Hot plugging, also referred to as
hot swapping (but see below), is the addition or removal of
hardware components from a
computer or other system while the power is on and the system is running normally. Until recently, the vast majority of
personal computer components could not be treated this way; the system had to be brought to
power off (or, in some cases, to some sort of
standby) before hardware could be exchanged. In order for a
computer system to support the
hot plugging of
peripherals, both the
software and
hardware must be designed with this in mind.
Many of the common hardware components on Intel PCs are not hot pluggable -- ISA cards, PCI cards, IDE/ATA disks, and most SCSI disks come to mind. Attempting to reconfigure these devices while the power is on will cause your computer to crash, and is likely to damage the hardware as well. Technically, PS/2 keyboards are not supposed to be hot-plugged either, but I have never encountered a system damaged this way. RS-232 serial devices such as modems and terminals, on the other hand, can be safely hot-plugged (given good cables).
More recently, several hot-pluggable peripheral interfaces have become popular: PCMCIA and CardBus, for laptop computer peripherals; USB, for low-bandwidth devices such as keyboards, printers, and still cameras; and FireWire aka IEEE 1394, for high-bandwidth devices such as digital video cameras and mass storage.
Server-class hardware has generally supported more hot-pluggability than desktop hardware -- often to allow for expandability or failover. Some classes of high-end server and mainframe systems allow the hot-plugging of CPU boards to add computing power or replace damaged boards. Even midrange servers now support hot-plugging of redundant power supplies, and of SCSI disks when fitted to a special backplane.
Still, the most common piece of hot-pluggable hardware remains one that nobody really thinks of as being hardware anymore: the removable-media storage device: floppy disks, CD-ROMs, and so forth.
Regarding "hot-plug" vs. "hot-swap": czeano has pointed out to me that in some contexts, "hot-pluggable" refers to the hardware's ability to survive being connected or unconnected while power is on, whereas "hot-swappable" refers to the running OS's ability to correctly register and unregister hardware components as they are plugged or unplugged. As an example, you might have a
Dell PowerEdge server with a hot-pluggable SCSI backplane, but be running an OS which scans the SCSI bus only at boot time. In this case, adding a SCSI disk to the backplane while the power is on is guaranteed not to damage the disk or the server, but you cannot expect to be able to mount the disk without a
soft reboot.