1. Galaxies are large collections of stars, gas, and dust held together by gravity and located in the vast spaces between galaxy clusters in the observable universe. 2. There are three main types of galaxies: elliptical galaxies which are spherical, spiral galaxies which have a central bulge and spiral arms, and lenticular galaxies which have properties between elliptical and spiral galaxies. 3. Galaxies were first cataloged in the late 18th century and important early catalogs include the Messier Catalog, New General Catalog, and Index Catalog which helped establish the field of extragalactic astronomy.