Folding Knives
A pocket knife is a folding knife with a blade that fits inside the handle and that is small enough to fit in a pocket. Blades are typically no larger than 3 to 5 in. (8 to 13 cm) in length. more...
Pocket knives are very versatile tools, and may be used for anything from opening an envelope, to cutting twine, to slicing an apple.
Pocket knives can cost anywhere from $1 (US) from gas stations and thrift stores, to thousands of dollars for custom, hand-crafted pieces. Well-made knives start at around $20.
Slipjoint knives
Most pocket knives for light duty are slipjoints. This means that the blade does not lock, but, once opened, is held in place by a spring device that allows the blade to fold if a certain amount of pressure is applied.
These knives often have more than one blade, including an assortment of knife blade types (serrated, plain edged, saws) as well as a myriad of other tools such as bottle openers, corkscrews, and scissors. A large tool selection is the signature of the Swiss Army knife. These knives are produced by Victorinox and Wenger and issued to the army and sold to the public. The German Army knife is large but light, with two blades opening from each side. It has hard plastic grips and aluminum liners. The United States Army knife, made by the Camillus Cutlery Company, used to have rustable blades and brass liners but is now all stainless steel and therefore very heavy and rugged. It has four blades opening from the same side. The handle, as manufactured, has rough edges, but these can be rounded, yielding an excellent and versatile knife.
Read more at Wikipedia.org