A galaxy can be roughly defined as a collection of stars, gas and dust and they are all gravitational bound as a single entity. These galaxies are the building blocks of our universe. In reality, we encounter a wide variety of galaxies. Some of them are elliptical in shape and are rather feature less. They are called the elliptical galaxies like the NGC 4150 galaxies shown here in the right. There are other galaxies too, which have a dysmorphology and often exhibit spectacular spiral structure. The world pool galaxy is a classic example of this kind of galaxies which is shown in the left. These are called dys galaxies. Since they have the spiral structure, often they are alternately referred as spiral galaxies. Now, to understand the population of galaxies in a better note, one needs to classify them based on some common characteristics they share. It was the great astronomer Edwin P. Hubble who first classified the morphology of galaxies in the local universe. The resulting classification scheme is popularly known as the Hubble Tuning Fog diagram. As the classification scheme, we are close resemblance with the tuning for structure, which is shown here. On the left, we have the elliptical galaxies and on the right, we have the dys galaxies. The current understanding is that our dys galaxy grows in size by marching with other galaxies and at the end, they become the ellipticals. Now our galaxy, the Milky Way, which hosts the solar system is also a dys galaxy, which has spiral arms. It can contain around 10 to the power 11 stars in it. Since we are residing within it, therefore you cannot see the full view and the picture shown on the left is merely an artist impression. Nevertheless, some part of the spiral arms of the Milky Way is feasible to us. There is a picture showing a part of the spiral structure. This picture is taken from the from from 80s, Neu Nithal India. Finally, there are estimated to be around 10 to the power 11 galaxies in the universe. Often they come very close to each other and therefore they pull or interact tidally. In the extreme case, one galaxy can get trapped into the gravitational potential of the other galaxy as a result. Finally, they march with each other. Here are some of the real images of the galaxy interactions where the galaxies are caught in the action. These images are taken from the European Southern Observatory's website.