It is a D shaped connector and has got 9 pins in it.
A video port has 15 pins (3 rows of 5 each) A video port, also known as a Serial Port, has 3 rows of 5 pins which is 15 pins.
The traditional parallel port is a 25 "d-sub" connector. Serial ports come in two flavors, either 25 pin "d-sub" or 9 pin. USB (another serial communication standard) is a different animal. PS/2 is also a serial port and it has six pins.
9
9
A DVI port utilizes a 24-pin connector.
If it has 15 pins it is NOT a serial port! A serial port will have either 9 pins (DB-9), or 25 pins (DB-25). The 15 pin connector (usually blue) is analog video or VGA, and yes if your graphics card is powerful enough it can run dual monitors. You can either buy a dual port graphics card or a splitter to run dual monitors
The connector is usually a DB9, though it's possible that not all 9 pins will be used.
Standard serial ports have either 5 pins or 25 pins. Although are also serial ports, USB and PS/2 ports do not obey this rule.
15 pins
As with ports, the gender of a connector is determined by whether or not it has pins. For example, a D-sub connector is male if it has pins (i.e. a VGA connector), whereas a serial connector is female (a serial port on the motherboard is male [it has pins to receive the connector], and the connector has the holes in which the pins are inserted). There is a reason they refer to the meeting of male and female plugs "mating"! Don't think of it as what's being plugged in and what's receiving the plug, but rather whether or not the connector and ports have pins or holes to receive the pins.
Go to a computer accessories shop and look for the green USB to PS2 Mouse adapter. Or you can buy it online. See the picture at this link (check that the port matches your PC mouse port): http://www.rudyp.com/usbtops2moad.html IMPROVED: Please note the answer above is for converting a USB to PS2 connector, NOT USB to Serial Port. A USB to Serial Port adapter does not, to my knowledge, exist. So please verify if you want a PS/2 (small, round connector) or Serial Port (long, flat connection with many pins) Also, the color of the adapter may vary.