LCD (Liquid Crystal Display) screen is an electronic display module. A 20×4 LCD display is very basic module and is very commonly used in various devices and circuits. These modules are preferred over seven segments and other multi segment LEDs. The reasons being: LCDs are economical; easily programmable; have no limitation of displaying special & even custom characters (unlike in seven segments), animations and so on.
A 20×4 LCD means it can display 20 characters per line and there are 4 such lines. In this LCD each character is displayed in 5×8 pixel matrix. This LCD has two registers, namely, Command and Data.