The four types of galaxies are spiral, barred spiral, elliptical and irregular. Each group spans large variations in size, number of stars, star density and general configuration.
Spiral galaxies are shaped like large flattened disks that spin around a central super-massive nucleus. Many of them are divided loosely into spiral arms, but a small percentage have no such structures and just appear to be featureless disks.
Barred spiral galaxies are similar to standard spiral galaxies, except that the spiral arms emanate from a large central column, or bar, that straddles the galactic core.
Elliptical galaxies have no discernible central core, and they appear to be ellipsoid, or spherical, in overall shape.
Irregular galaxies appear in many chaotic shapes and have little to no overall internal structure.