DE-9 (sometimes incorrectly called DB-9) is a D-shaped connector/port with two rows totaling 9 pins in an E-size connector. The top row has 5 pins, and the bottom row has 4 pins. Due to the fact that it is a serial port, transmission is done one bit at a time. Two types of DE-9 connectors exist. A male DE-9 connector has pins, while a female DE-9 connector has holes. You can only plug a male connector into a female plug or vice versa, unless you have a DE-9 gender changer.
DE-9 ports are commonly seen on PCs as RS-232C serial ports, but were also featured on every Sega home console from the SG-1000 to the Mega Drive for connecting joypads or other input devices. During the 1980s and early 1990s, it was an unofficial standard for gamepads and joysticks, meaning controllers were interchangeable between the various computers and consoles of the time (though there might be some compatibility issues due to how each hardware may respond to button inputs differently). This has since been phased out in favor of proprietary controller ports (as was the case with the Saturn and Dreamcast), or in the case of computers and more recent consoles, has largely been replaced with the USB standard.