A 2 pole connector available in several sizes for DC power input applications. The plug consists of a 'barrel' with a hole in the middle, the 'pin' actually being on the socket part. The connectors push together. Chassis sockets normally include a switch which is operated when the plug is inserted, to disconnect batteries when a mains adapter is connected. They are only suitable for use at low voltages. The connectors are available in 1.3mm, 2.1mm and 2.5mm sizes, the measurement referring to the diameter of the pin on the socket.
Personal Stereos and CD players (1.3mm type)
Computer equipment with separate mains adapters
Battery powered equipment with a connection for an AC adapter.
Can be used with either centre connection positive or negative depending on equipment, although normally the centre pin is positive and the outer sleeve negative. There may be a diagram next to the input socket to show which polarity to use (most AC adapters have a switch or reversible plug to set the polarity). For home-built equipment it is probably best to stick to centre positive since this seems to be the most common.
Normally solder connections. Be sure to slide the cover onto the cable before soldering! There are normally a pair of metal tabs to clamp the cable with - squeeze these shut with pliers AFTER soldering (they will probably melt the insulation if you clamp the cable before you solder).
Normally solder connections, Switched versions will have 3 terminals - the positive, negative and the break contact. This contact will be connected to the sleeve terminal when the plug is out, and open-circuit when the plug is in. You will need to refer to the suppliers catalogue or use a multimeter to check which terminal is which.