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.