The galactic disk is the the highly flattened area of the Milky Way galaxy, and it contains spiral arms which come out of the center. Stars in the galactic disk are a mixture of old and new, and the movement of these stars is relatively predictable, thanks to the strong gravitational pull coming from the center. The spiral arms themselves do not wrap around the galaxy in a circle formation because stars on the outer end die out before they rotate back around.
The center of the Milky Way galaxy is also called the galactic bulge. Stars in the bulge do not move in a spiral around the Milky Way, but instead have a more random orbit that keeps them closer to the center. The closer a star is to the Milky Way's bulge, the more likely it is to be a younger star. As stars grow older and large enough to escape the gravitational pull in the center of the Milky Way, they often end up in the arms.
The galactic halo is the area of space which surrounds the exterior of the Milky Way. There are no young stars in the galactic halo, and the oldest stars in the halo are believed to be around 10 billion years old. The halo is believed to be primarily made up of dark matter, which is matter that is not visible with any known form of detection. Stars in the halo move in a random orbit around the Milky Way.
The actual center of the Milky Way is still debated by scientists. There is a force that has the mass of about 4 million stars that appears to keeps stars directly in the center in orbit, and some scientists believe that this could be a super massive black hole. A black hole is a point in space so gravitationally dense that not even light can escape it. Thus, like dark matter, black holes are difficult to detect, however, the orbit of the stars in the center, coupled with the radiation given off in the center, leads credence to the black hole theory.