The external tank is a major part of the space shuttle system. It is the main fuel source for the shuttle's three main engines and is separated after 8 minutes of flight.
The fuel tank was painted for the first 2 flights white but the white paint was way too expensive so the engineers decided to paint the external tank the orange color orange we know today.
The SLS is now using a shuttle-like fuel tank as the core stage of the rocket.
Before the shuttle was launched engineers at NASA had a wierd idea. They decided that the fuel tank would have a payload fairing but the plan was scraped and never used again.