What exactly is emulsion paint? I have never heard of it.
A water-based paint where the paint is "emulsified" into the water. Used mainly for painting walls etc and usually comes in matt or semi-gloss variants. Some DIY stores here in the UK will mix up customers' own colour specifications, so for example you can match GW's Goblin Green should you wish. I'm sure you have the same thing in the USA, but under a different name.
It's like "spackle". We have similar stuff here, but not under that name