It's a spherical star collection that orbits a gallactic core, the rotational center of the milky way.
The globular clusters have a spherical shape because they are thightly bound by gravity.
How do globular clusters form?
The formation of globular clusters is still not a well known proces, but we do know somethings.
They do ussually get formed at some regions named starbust, ther's a correlation between the mass of a supermassive black hole, situated at the center of a galaxy, and the extension of the globular clusters from the elliptic galaxies and lenticular galaxies.
We don't know any globular cluster that shows the formation of new stars. That gives mor credit to the opinion that globular clusters are usually the most ancient objects in the galaxy.