Galaxies

A galaxy is a huge group of stars, dust, gas, and other celestial bodies bound together by gravitational forces. There are billions of Galaxies in the Universe. Typical galaxies range from dwarfs with as few as ten million stars up to giants with one trillion stars, all orbiting the galaxy's center of mass. Galaxies may contain many multiple star systems, star clusters, and various interstellar clouds.
The Earth, Sun and the rest of our solar system are a tiny part of the Milky Way Galaxy, a spiral galaxy. The Milky way Galaxy is just one galaxy in a group of galaxies called the Local Group. Within the Local Group, the Milky Way Galaxy is moving about 300 km/sec (towards the constellation Virgo).
The galaxy that is nearest to our galaxy is the Sagittarius Dwarf galaxy, which is about 24 kiloparsecs or 80,000 light years from us. The Large Magellanic Cloud is another close galaxy; it is about 50 kiloparsecs from us.
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. Although it is not yet well understood, dark matter appears to account for around 90% of the mass of most galaxies. Observational data suggests that supermassive black holes may exist at the center of many, if not all, galaxies. They are proposed to be the primary cause of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy appears to harbor at least one such object within its nucleus.
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 very unlikely
There are three major types of galaxies: spiral (with arms), elliptical (no arm), and irregular (without rotational symmetry). The only difference between the three is what shape they are.