Quick question: What was the OLD Galactic Catalog?
And what is their purpose?
Nitpick: it’s the New General Catalogue.
And it superseded William Herschel’s Catalogue of Nebulae.
(Info from the two Wikipedia articles linked to above.)
DON’T PANIC.
The NGC is a comprehensive catalogue of all deep sky objects including nebulas, galaxies, and others kinds of clusters, in which these objects can also be abbreviated as NGC as well to only further confuse matters. We blame the Vogons.
No nitpick, Giles, you’ve fought my ignorance well. 
Basically, there are just way too many things in the sky to give proper names to all of them, so we have to give them dry numerical labels instead, and the NGC is just a big bunch of those labels.