MIDI stands for "musical instrument digital interface." The MIDI Manufacturers Association explains that it "is a system that allows electronic musical instruments and computers to send instructions to each other." It is a standard type of technology, not the actual sound that travels between devices, as some people mistakenly think. Instruments, synthesizers, computers, cellphones, software programs and connectors cables are some common products that use MIDI as part of the process in creating, sharing and recording music.
MIDI-enabled devices are designed to communicate with one another. When someone performs one operation on one device, it can tell another device what it is doing. For example, electric keyboards, guitars and drums are called controllers. When you use them, they send messages to a device such as a synthesizer, which then outputs a corresponding sound depending its settings. Besides digital music creation and playback, MIDI has other interesting uses such as controlling rides at theme parks and operating themed events outside of Las Vegas casinos.
MIDI devices communicate with one another over connectors called DIN cables. DIN stands for "Deutsches Institut für Normung," which means "German Institute for Standardization" when translated. These cables are round and have pins arranged in a circular pattern. They connect MIDI devices together through "MIDI IN" and MIDI OUT" ports. Devices can also be daisy-chained with these cables by using "MIDI THRU" ports. DIN cables come in a variety of sizes with different numbers of pins. Most MIDI devices use five-pin cables.
The industry standard for connections between MIDI devices is five pins. The reason for standardization is so that multiple types of devices can communicate with each other using the same type of equipment. Connectors and ports may contain a different number of pins, but they generally won't be very useful with a wide range of MIDI products. If you have a device with eight pin holes in its MIDI port or ports, an adapter would give you the most versatility for your device.