Question

There are billions of galaxies in the universe. How are they classified?

Answer 1

Galaxies are classified into four classes: spirals, barred spirals, ellipticals and irregulars. It is based on the shapes of the galaxies.

Explanation:

Galaxies are very important fundamental building blocks of the Universe. Some are simple, while others are very complex in structure. As one of the first steps towards a coherent theory of galaxy evolution, the American astronomer Edwin Hubble, developed a classification scheme of galaxies in 1926.

The Hubble classification of galaxies, also referred to as the ‘tuning fork’ diagram because of its shape, classes galaxies along three main lines into:

Elliptical galaxies
Spiral galaxies
Barred Spiral Galaxies

Check this out for a super long list of different types of galaxies: https://astrobites.org/guides/galaxy-and-agn-types/

It is a really vast topic. Here are a few links that might help you out:
https://www.spacetelescope.org/images/heic9902o/
http://pages.astronomy.ua.edu/keel/galaxies/classify.html