The Triangulum and the Andromeda galaxies are examples of this type and observations from the Spitzer Space Telescope suggests, for 10 points, that our own galaxy might be a barred form of this type of galaxy which notably includes arms.
One theory of their formation was put forth in 1964 by Lin and Shu. A empirical relationship which defines the luminosity of this object to the amplitude of its rotation curve is given by the Tully-Fisher relation. Hubble first described these objects, suggesting that a central concentration of stars creates a characteristic "bulge." The Triangulum and the Andromeda galaxies are examples of this type and observations from the Spitzer Space Telescope suggests, for 10 points, that our own galaxy might be a barred form of this type of galaxy which notably includes arms.
One theory of their formation was put forth in 1964 by Lin and Shu.
{spiral} galaxy [prompt on partial answer]