Whats the difference between a star cluster and a galaxy?

It's a good question... Star clusters are usually associated with galaxies - they're smaller, the stars have usually formed from a single nebula, so they're 'sister' stars (in many cases...) Galaxies - even the irregular galaxies - usually have far more stars than what we would regard as a 'cluster.'

There are WAY fewer star in a cluster, maybe only a few million.

There are often several hundre million, even billions, of star in a galaxy.

There's some overlap among small galaxies/large globular clusters, but basically galaxies are bigger, often much bigger.