There are billions of Galaxies in the Universe. Some are very small with only a few million stars. While others could have as many as 400 billion stars, or even more. There are three kinds of Galaxies, Spiral , Elliptical , and Irregular . The only difference between the three is what shape they are.
Galaxies often crash into one another.
Even our own galaxy has had others pass right through it. Don't worry though, galaxies can pass through each other quite safely. Stars are so far apart that the chances of two colliding is nearly impossible. Their long twisting arms are areas where stars are being formed. The spiral arms seen in this kind of galaxy are circling waves, much like the ripples of a pond.