While the tendency is to focus on guns when discussing illegal weapons, there are also certain kinds of knives that are illegal to possess. Depending on where you live, some kinds of knives may be classified as illegal weapons or will require a permit to lawfully carry.
The most widely-prohibited knives are switchblades. Switchblades are knives that fold in and can be opened by a mechanism such as a button or lever. These weapons can be opened with one hand and spring into a usable blade in the blink of an eye. The blade swings into a locked position when opened.
Similarly, knives that are opened via centrifugal force are widely illegal as well. Known as gravity knives, these blades can easily be opened using one hand as well. Bladed throwing weapons are also commonly illegal, as are pointed blades that serve no purpose other than stabbing.
There are a lot of regulations in place concerning which kinds of knives are legal and illegal. The federal Switchblade Knife Act describes which weapons are illegal on a nationwide level, with further restrictions imposed on a state-to-state basis. Familiarize yourself with your state laws in order to prevent inadvertently being charged with possession of a deadly weapon.