They differ from other spiral galaxies in that have nuclei that are shaped like bars instead of spheres. As with many objects in the universe barred galaxies are not fully understood. No one is exactly sure why these galaxies have bars or how they form. There are also many theories as to what happens inside of these bars. The subject of barred galaxies hits close to home since many scientists now believe that our own galaxy, the Milky Way, is a barred.
Spiral galaxies are disc shaped galaxies that have distinct spiral arms.
These arms are actually density waves. In these areas the galactic material has slowed. Other material such as gas and dust that is circling the galaxy runs into these slower more dense arms. This occurrence can be compared to traffic jams. Once this gas and dust collides with the slower moving material it forms new blue stars.