How are galaxies star clusters and nebulae different?
Galaxies contain star clusters and nebulae.
A nebula is a cloud of gas and dust, mainly hydrogen. They can be formed by gravitational collapse of interstellar medium which is mainly hydrogen and helium. A nebula can also be formed from the remnants of a supernova explosion of a dying star. Nebulae are often star forming regions where new stars are born.
Star clusters are collections of stars. Globular clusters are between 10 and 30 light years across which typically contain between 10,000 and several million stars. They are normally old stars. Open clusters are collections of up to a few hundred stars. These are normally young stars.
A galaxy is a collection of objects bound together by gravity. They include stars, star clusters, gas and dust. They are also believed to contain dark matter.