Romex is a type of electrical wire that’s mostly used in homes and buildings. The term electrical wire is just more broad and used in the electrical field. There are a few different types of wire in that field like Romex, UFB cable and THHN wire.
THHN wire is a one conductor bare copper wire with a PVC jacket and nylon coating. The most popular awg sizes from my experience are THHN 12 and THHN 4/0. The PVC jacket is the cheapest type of insulation because it doesn’t need any more than that. The nylon coating is for moisture protection and it has a small amount of lubrication for easier installation in conduit. THHN wire can be used indoors, outdoors or in conduit. It cannot be used directly in the ground. Many people ask to use THHN wire as dog fence wire and it doesn’t work.
Romex wire is very similar because it’s a cable filled with two or three THHN wires and a bare copper ground wire. The Romex 14/2 and Romex 6/3 have proven to be the most popular awg sizes based on volume. Romex only has a very light PVC jacket over it so it can’t go outdoors or in conduit at all.
How can THHN wire go outdoors, but then THHN with an additional PVC jacket can’t? The reason is based on the ground wire not the THHN. The ground wire doesn’t have any insulation so if moisture penetrates the outer jacket than it will directly hit the bare copper. Romex cable was made to run more than one conductor at a time along with a ground wire.
The next upgrade is to UFB cable which is a very popular type of electrical wire. It’s the kind of cable that can do almost anything. It was made to be outdoors, in conduit and directly underground. So you might also hear it referred to as underground cable. UFB 6/3 and UFB 8/3 are the most popular awg sizes of underground cable because you’ll usually need a bigger awg size when running the wire outdoors.
As you would probably think, each type of electrical wire gets more expensive as you upgrade because they perform more functions. Most times the wire or cable doesn’t have to go outdoors so the Romex wire is the most popular. The UFB cable isn’t that much more expensive than the Romex because it’s just a different insulation and the biggest cost is the copper inside.