Today romex is the most common type of wiring used. Romex is a composite cable consisting one hot conductor(Black), a neutral conductor(White) and a ground wire(Copper). All the conductors are individually insulated and the entire bundle is sheathed in PVC plastic to make a nice neat package. Romex is flexible, heat and fire resistant, easy to install.
BX is also known as Armor Clad or flexible metal conduit cable. BX provides all the advantages of Romex plus some additional benefits. It resist puncture by nails and screws and more resistant to be chewed through by rodents and other pests. Comercial applications use also individual conductors inside conduit. This is a more difficult and time consuming wiring method, but is more efficient where wiring must be exposed.