Galaxies are huge collections of stars. Light takes a fraction of a second to go from any point on earth to any other point. This is why when we talk via satellites, there is no real noticeable delay. To speak about the sizes of galaxies, we speak about the time it takes light to go from one end to another. This is usually about 100,000 years.

 

Here is a picture of a typical galaxy.

 

 

Our galaxy, the Milky Way, looks like this, if we can imagine ourselves far away and looking at it.

 

Note that it seems to be rotating about the center. Most galaxies have a huge black hole at the center. The stars go in orbit about the black hole in a fashion similar to the orbit of the earth around the sun.