Hubble's scheme divides galaxies into 3 broad classes based on their visual appearance:
Elliptical galaxies are classified according to how squashed they look. Circular ellipticals are classified as E0, and the more squashed an elliptical looks, the higher the number it is given, such that the most flattened ellipticals are classified as E7.
Given the tag S0, these galaxies consist of a bright central bulge surrounded by an extended, disk-like structure. Unlike spiral galaxies, however, the disks of lenticular galaxies have no visible spiral structure and there is little evidence of star formation.
Standard spiral galaxies are given the symbol S, but as at least half have a bar-like structure (barred spirals), they are given the symbol SB. These in turn are arranged into three sub-categories based on the tightness of the spiral arms and the size of the central bulge. Sa and SBa galaxy types feature tightly bound arms and a large central bulge, whereas Sc and SBc galaxies have loosely bound arms and a small central bulge. Sb and SBb galaxies lie somewhere in between.
These broad classes have been extended in other classification schemes to enable finer differences in appearance, and to encompass other types of galaxy, such as irregular galaxies, which appear to have no obvious structure. Some of these have been caused by galaxies colliding with each other.