Galileo is famous for many things but is probably best known for discovering Jupiter's four moons. The moons around Jupiter are called "Gallilean moons."
Besides finding Jupiter's moons, Galileo investigated the laws of motion and made improvements to the telescope. He questioned, what many scientists at the time believed, the universe revolving around the Earth.
Galileo was the first scientists to study the universe with a telescope, and he built telescopes to earn money. He discovered that Venus had phases like Earth's moon and discovered the rings of Saturn.
Galileo's theories were not widely accepted until after his death. He spent the last 8 years of his life under house arrest, and the Catholic church refused to print any of his findings.