The concept of Galaxy comes from the latin galaxias which, in turn, is borrowed from a Greek term. According to the Larousse dictionary online, a Galaxy is a large set formed by a variable amount of stars, planets, interstellar (or cosmic) dust, gas and particles.
In galaxies there are several substructures such as nebulae, star clusters and multiple stellar systems. From Earth, all the stars that are visible at simple view belong to our own galaxy, the milky way.
The stars that are part of a Galaxy maintain a gravitational interaction and orbit around a common centre (in the case of the milky way, the Center is the Sun).
Intergalactic space is formed by a gas that has an average density less than the atom per cubic meter. In general, the galaxies have several associations known as clusters which, in turn, can form larger associations (Supercluster).
Galaxies may have different forms. That said, it is worth mentioning the irregular galaxies (they have disturbances generated by the gravitational attraction of nearby galaxies), elliptical galaxies (they have the shape of an ellipse) and spiral galaxies (they have a circular shape and curved arms wrapped dust), for example.
The milky way belongs to a group of about 40 galaxies, namely, the Local Group. This group has a weight of 1012 solar masses and an average diameter of about 100,000 light years. Scientists believe that our Galaxy contains between 200,000 and 400,000 millions of stars.