Butterfly knives are also referred to as balisongs, fan knives, butterfly knives or Batangas knives. They are a form of folding pocket knife which has two handles rotating around the tang such that, when the knife is closed, the blade is concealed between grooves in the handles. Historically, they are associated with the Tagalog region of the Philippines.
There are two main types of construction for butterfly knives: sandwich construction and channel construction.
In sandwich constructed knives, there are generally layers pinned or screwed together, which allows pivot pins in the knife to be adjusted more easily without binding. The blade rests between the layers.
Channel constructed butterfly knives are made with the main handle from one piece of material, and a groove is created within the piece of material for the blade to rest.
These knives are restricted in some countries, and state regulations vary widely, so make sure to check out the rules where you are before purchasing a butterfly knife.