There are different types of wiring present and used in residential homes and buildings today.
Knob & Tube Wiring
This type of wiring was used in buildings in North America from about 1880 to the 1930s. It consists of single-insulated copper wires run within walls or ceilings, passing through joists and stud drill-holes through protective procelain insulating tubes, and supported along their length on nailed-down porcelain knob insulators. Due to the age of this wiring, if there are an live wires in your home or building, the should be replaced immediately. This type of wiring is also known to cause problems with squirrels and other rodents damaging the wires.
BX wiring are bundles of insulated wires sheathed in a flexible and protective metallic sheath.
BX or Type AC is one of the earliest types of electrical cable made for both residential and commercial uses in the early part of the 20th century. "BX" is the older term for this type of cable. It’s assumed that the name originated from the Bronx, NY since the first BX cable was made there.
BX cable is still used today in both residential and building applications. It is especially used when the wiring is exposed and not protected by interior walls i.e. workshops, commercial buildings, etc.
If the protective insulation is nicked or cut, the wiring should be replaced. Also, if the wires within show degrading of their rubber insulation, these wires should also be replaced.
The word Romex® refers to any type of non-metallic sheathed electrical cable. It is electrical wiring sheathed in a plastic coating.
The name comes from the Rome Cable Corp. of Rome, NY, which originally produced the wire. Now the Romex® brand is owned by Southwire and is an actual trademarked brand just like Jell-O or Kleenex. This type of wiring is the most commonly used in homes and buildings today.
As always, you should have any electrical repairs, alterations or inspections performed by a qualified, licensed electrician.