A galaxy is simply a vast collection of dust, gas, and millions of galaxies collectively with their own massive stellar host stars and their own stellar centers. The Milky Way galaxy, which is a spiral arm of large spiral arm galaxy also known as the Virgo cluster. A galaxy holds together by gravity pulling on itself into a shape that is similar to a football.
Now if you study a galaxy like the Milky Way you will see thousands or even millions of stars rotating together as the nucleus of this massive structure. While the Milky Way is only about 50 million light years in width, a galaxy of the same size could be several hundred or even millions of light years in width with numerous intervening spiral arm regions between the stars. A galaxy is also held together via gravity by the black hole at its core. When you look at stars at night in the night skies, you are looking up at other, faraway stars within the Milky Way galaxy. If you were to go to an observatory that can be used to study these nearby galaxies, you would see a faint glow of light that would indicate that another galaxy, much bigger and older than the Milky Way, is orbiting the very inner region of the Milky Way galaxy.
This is all fun fact to those who have a keen eye for details. While it is all well and good to know that many galaxies are just collections of stars, it is fascinating to know that some of these close spiral arm galaxies that we study are actually a way of interacting with other such gigantic spiral arm galaxies. For example, although a very close spiral arm is found in our own galaxy called the Milky Way, it is also a member of an extended system of galaxies which is actually a member of a larger family of galaxies called the Open Galaxy System. In fact, according to theories by astronomers, our Milky Way is actually part of a super cluster which is a complex cluster of multiple galaxies which lie far away from the centers of the Milky Way Galaxy and are therefore also not members of the Milky Way Galaxy. This is one of the most widely-accepted theories on why galaxies are apart and why they interact so frequently with other similar clusters of galaxies.