This document discusses seven segment displays and how to interface them with microcontrollers. A seven segment display uses 7 LED segments (a, b, c, d, e, f, g) plus a decimal point to display numbers from 0 to 9. There are two types: common cathode, where the cathodes are connected and the anodes are individual; and common anode, where the anodes are connected and the cathodes are individual. When interfacing with a microcontroller like the 8051, common cathode displays use logic high outputs while common anode uses logic low outputs to light the correct segments to display the desired number.