This is called a Karambit or Kerambit. Many cultures in Asia have an implement with a similar shape as this weapon. Originating in Java the kerambit eventually reached what are now Cambodia, Laos, Malaysia, Myanmar, the Philippines and Thailand.
Some cultures use blades shaped similar to this as farming tools; primarily used to cut rice and other crop foods for harvest. Necessity has occasionally forced these farmers to adapt the use of these blades and other farm tools for the purpose of self defense.
The karambit has also found a home in popular cinema. You have seen this versatile knife in movies like Ong Bak 2, The Punisher, and the movie "Taken" starring Liam Neeson.
With proper training this blade is devastating. Close quarter or with an extended reach, the Kerambit can deliver powerful slashing cuts or "rips" as well as serve to control an attacker by grabbing on to bones and appendages.
Like I always say regarding any weapon; A timid user equals a timid weapon. So if you plan on using a weapon of any kind for self defense, invest in proper training by a qualified instructor, then when the time comes to defend yourself, like they say in poker...All In!!