PCs may have from one to four COM (serial) ports. If present, these ports are often associated with sockets on the back of the PC. COM ports are used to connect serial devices, such as a mouse, modems, PCMCIA sockets, uninterrupted power supplies and other computers to the PC. Older PCs often have one or two serial sockets on the back that are associated with COM ports. Often the 3rd (and 4th) COM port(s) is accessed from the operating system though one of the slots on the motherboard (if they exist on your system).
Devices such as modems may be installed internally, but they still make use of a COM port even though they are not physically connected to a serial socket on the outside of the PC. In these the PC redirects the COM port via the PC slot to the card. If the device uses a COM port that is normally associated with one of the sockets on the back of the PC, the port is taken away from socket and given to the device. The implications of this will be clear in a moment.
Each COM port gets the attention of the computer's processor by means of an Interrupt Request, called an IRQ. Think of these as "channels" by which the COM ports tell the processor that there is data waiting. There are only 15 IRQ's available to standard serial devices, and several are already dedicated internally. Because of this, COM ports 1 and 3 (if installed) share IRQ4, and COM ports 2 and 4 (if installed) share IRQ3. This usually means that if there is a serial device, such as the mouse, using COM1, and another using COM3, such as a modem, the two devices will experience an "IRQ conflict" which is likely to disable both. These conflicts must be resolved before the devices can be used successfully.