The countless stars in the Universe are not uniformly distributed in Space. They are found in huge clusters. A large Group of Such Stars, Nebulae(cloud of dust and gas, in which star is born) and other heavenly bodies bound together by their own Gravity is called a Galaxy.

A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. 

In fact, galaxies are the building blocks of the Universe.

According to One estimate, there are about 1011 galaxies in the 

Universe. On an average each galaxy has 1011 stars in it. 

Depending on their shapes,galaxies can be Spiral, Elliptical, Lenticular or Irregular.

Types of Galaxies 

In 1936, Hubble debuted a way to classify galaxies, grouping them into four main types: spiral galaxies, lenticular galaxies, elliptical galaxies, and irregular galaxies. More than two-thirds of all observed galaxies are spiral galaxies.

Spiral Galaxies

A spiral galaxy has a flat, spinning disk with a central bulge surrounded by spiral arms. That spinning motion, at speeds of hundreds of kilometers a second, may cause matter in the disk to take on a distinctive spiral shape, like a cosmic pinwheel. Our Milky Way, like other spiral galaxies, has a linear, starry bar at its center. 

Elliptical galaxies 

Elliptical Galaxies are shaped as their name suggests: They are generally round but can stretch longer along one axis than along the other, so much so that some take on a cigar-like appearance. The universe’s largest-known galaxies—giant elliptical galaxies—can contain up to a trillion stars and span two million light-years across. Elliptical galaxies may also be small, in which case they are called dwarf elliptical galaxies.
Elliptical galaxies contain many older stars, but little dust and other interstellar matter. Their stars orbit the galactic center, like those in the disks of spiral galaxies, but they do so in more random directions. Few new stars are known to form in elliptical galaxies. They are common in galaxy clusters.

Lenticular galaxies

Lenticular galaxies, such as the iconic Sombrero Galaxy, sit between elliptical and spiral galaxies. They’re called “lenticular” because they resemble lenses: Like spiral galaxies, they have a thin, rotating disk of stars and a central bulge, but they don’t have spiral arms. Like elliptical galaxies, they have little dust and interstellar matter, and they seem to form more often in densely populated regions of space.

Irregular galaxies

Galaxies that are not spiral, lenticular, or elliptical are called irregular galaxies. Irregular galaxies—such as the Large and Small Magellanic Clouds that flank our Milky Way—appear misshapen and lack a distinct form, often because they are within the gravitational influence of other galaxies close by. They are full of gas and dust, which makes them great nurseries for forming new stars.