He identified the main types of galaxies as spirals, barred spirals, ellipticals, and irregulars. He came up with what is now called the tuning fork diagram. Since Hubble's time we have also added two new types, the lenticular and barred lenticular. At first it was thought that this diagram represented some sort of evolutionary sequence. Later the idea was discredited, but now it is making a comeback in a limited way, as we'll see later on.
Spiral galaxies have a disk and a bulge, and spiral arms in the disk.
Spirals with large bulges and tightly wound, smooth featured arms are called Sa's, those with small bulges and loosely wound arms with lots of structure are called Sc's, and intermediate spirals are called Sb's. Barred spiral galaxies are like spirals, but with a bar-like central feature. They are also subdivided into SBa, SBb, and SBc types. Elliptical galaxies are just big, ovoid blobs of stars. Most of them are featureless.