THC oil, marijuana oil, and THC vape juice are generally associated with the same product. Essentially, they all mean the same thing. With the help of alcohol or CO2 extraction, this oil is made by extracting the resin of the female marijuana plant.
Remember, the marijuana plant is the high-THC strain. The resin dissolves in alcohol and then the alcohol evaporates. The residue will consist of a thick syrup abundant in THC. This type of oil is only legal in states which have medical marijuana laws or have implemented recreational laws regulating Brass Knuckles.