Stars are huge luminous balls of gas powered by nuclear reactions at their centers. The enormousy high temperatures and pressures in the core of a star force atoms of hydrogen to fuse together and become helium atoms, releasing tremendous amounts of energy in the process.
Planets are much smaller with core temperatures and pressures too low for nuclear fusion to occur.
Thus they emit no light of their own. When you see Venus or Jupiter in the night sky, you're really seeing sunlight reflected by those planets back to you. Some planets, like Earth and Mars are solid rocky bodies, but others, like Jupiter and Saturn are mostly gas and liquid. Jupiter, the largest planet in our solar system, is roughly 300 times more massive than the Earth, but only one-thousandth the mass of the Sun. However, had Jupiter been 75 times more massive, it would just have been large enough for the pressures and temperatures at its core to ignite nuclear fusion, and the Earth would have had two Suns in our skies.