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Shape
Spiral galaxies are, of course, shaped like spirals with a large bright mass at the center and long "arms" of stars radiating out from the middle.
Formation
Spiral galaxies form from spinning clouds of hydrogen gas. Other types of galaxies also form from hydrogen gas clouds, but the clouds spin especially fast to form spiral galaxies.
Classification
Spiral galaxies are classified on the basis of the ratio of their center mass to the mass of their arms. Galaxies with large bulges and small, tightly wound arms are classified as Sa galaxies, while those with small bulges and loosely wound arms are classified as Sc or Sd. Some spiral galaxies have a bar-shaped central bulge and are called barred spiral galaxies. These are given the designation Sb and are quantified Sba through Sbc.
Composition
The bulge of spiral galaxies are composed of older stars while the arms are made of younger stars, insterstellar gas and dust.