The Hubble Tuning Fork
After he discovered what galaxies really were, Edwin Hubble became the first person to classify galaxies. Astronomers use his system, called the "Hubble Tuning Fork," even today. First, Hubble divided the galaxies into two general categories: elliptical and spiral galaxies. Elliptical galaxies are shaped like ellipses, and spiral galaxies are shaped like spirals, with arms winding in to a bright center.
Elliptical galaxies are classified by how round or flat they look. An E0 galaxy is very round and an E7 galaxy is very flat. In detail, the number after the "E" is determined by the galaxy's ellipticity - the ratio of the ellipse's major axis to minor axis. Galaxies with higher ellipticities have higher numbers.
Hubble noticed that some spiral galaxies have a bright line, or bar, running through them. He called these galaxies "barred spiral galaxies." Galaxies with spiral arms, but without the bar, are just called "spiral galaxies."
Spiral galaxies are further classified by how tightly their arms are wound. Type a galaxies have their arms wound very tightly and have large central bulges. Type c galaxies have their arms wound loosely and have small central bulges.
Some galaxies are a transition type between the elliptical and spiral galaxies, labeled S0 on the tuning fork. These are called "lenticular galaxies." Lenticular galaxies have a central bulge and a disk but no spiral arms.
The third class of galaxies is irregular galaxies. Irregular galaxies are neither spiral nor elliptical, and can have any number of shapes. They are frequently the product of two galaxies colliding with each other, or at least affecting each other through the force of gravity.