A galaxy is a group of stars, interstellar gas, dust, etc.
There are different types of galaxies. They are all categorized by their shape:
Spiral: a spiral galaxy has a disc in the centre and curved arms that come out of it. Irregular: it does not have a specific shape. It is just a mass of things. Elliptical: an elliptical galaxy looks basically like a disc in the universe.
I hope it helps. I tried to explain it as easy and as simple as possible!
