ADB

From Mac Guides

ADB (Apple Desktop Bus) was an analog connection used in early Macintosh computers manufactured before 1998. ADB was used for connecting keyboard, mice, joysticks, drawing tablets and other input devices to the computer. The use of ADB ended with the PowerMac G3 300Mhz (Blue and White) being the last PowerMac to include it, although the machine had USB ports and came bundled with a USB keyboard.

One of the advantages to ADB was that it allowed force restarts under hard crash situations and powering on the computer from the keyboard unlike USB which requires the computer to be running. ADB was also used in certain Apple CRT Monitors to calibrate the screen's position or color. For these monitors, you can not fully calibrate the monitor correctly on computers without ADB.