Hubble proposed that galaxies be classified according
to their appearance.
Sa: Large central
bulge, smooth, tightly wrapped spiral structure
Sb: Less noticable
bulge, looser spiral structure
M31, the Andromeda Galaxy
Sc: Weak or no
bulge, open spiral structure, very knotty appearance
M33, the Pinwheel Galaxy
There is a parallel sequence for spiral galaxies that have central bars: SBa, SBb, SBc
|The Milky Way is probably an Sbc or SBbc, something like this galaxy, M83.|
Elliptical galaxies are not just round. Hubble classified them as E0 -
E7, where the number refers to their axis ratio 1-(b/a).
[E0=round, E7=very squashed].
But note that the observed shape may have nothing to do with the true shape, due to projection effects:
From morphology alone we can't tell if elliptical galaxies are
Like the Large and Small Magellenic
Clouds, nearby companions to our own Milky Way
Hubble put these all together into his famous "tuning fork" diagram of galaxy classification:
This was often thought to represent an evolutionary diagram. It's not!
Here is a discussion of the pitfalls associated with using Hubble typing to discuss galaxy evolution (courtesy Greg Bothun, University of Oregon)