Sockets are female connectors -- holes
Plugs are male connectors -- pins sticking out.
Bus mice use the same type of mini-DIN socket.
At times found on the motherboard, these ports will also show up on I/O expansion cards, and Hard Drive Controllers (where they are often added as a sales "extra").
These are used for: modems, scanners, serial printers, mouse, graphics boards, null modems...
These ports are controlled by a URT chip. There may be a great deal of variance on how well any of these work for different applications, although, for example, a mouse wouldn't care much.
A modem, on the other hand, would want a "better" URT chip. If you nose around on the I/O board, or the HDC board, you may be able to find the URT chip. There are some five models in use.
Note that internal modems all use their own 16550A URT chips. No worry.
Note that that old MDA boards will also operate at composite video, so that Video Monitors with "video-in" (RCA) plugs can be used. There are adapters from the DB 9 plug of the MDA video adapter to an RCA plug (Pins 7 and 1 are used, pin 1 is ground).
Hercules monitors will work with MDA boards, or with Hercules boards. Hercules is B/W (orange on black, or green on black), and will do graphics, although slowly. You may have to set some motherboard jumpers and CMOS settings in order for it to work.
If the Hercules monitor is the only video card, it will be used by the system (although the BIOS may complain during boot-up). If used in conjunction with any other video card (MDA, CGA, EGA, VGA), the Hercules monitor can be used as a scratch pad to post information to while you do graphics work on another monitor.
It may be used for a printer, or an exterior Zip drive, scanner, or other specialty equipment. It is called a "parallel" port because data is sent and received on 8 lines at a time (rather than the serial ports, which only use one line for data out, and one for data in).
At the printer the parallel port cables often use a "Centronics 36" -- a strange looking connector the size of a DB 25 but with two rows of teeth, and wire clips at the left and right. These are being replaced by smaller connector which looks like a sized down DB25, with locking clips on the side.
Coax connectors for networking along a bus.
RJ-45 telephone like connectors (but wider) for networks on 10BaseT hubs.
DB 15 with a sliding lock: also used on networking cards.
50 pin and 68 pin SCSI connections. These look like DB 25 type connectors, but less wide, and with locks at both ends for the cables.
See also this text file on [pins-out].
You probably have only one Video socket, but could have up to four serial ports (DB 25 or DB 9) and three parallel ports (DB 25).