Triangle whisks are a type known as "gravy", "roux" or "sauce" whisk. The way the whisk is constructed creates a flat end. This flatness is useful for liquids in shallow pans.
This type of whisk often has four to six wires (tines) that each form a triangle. These tines are all roughly the shape size and they are laid out in a manner that the tines cross each other forming a whisk that looks quite typical other than the dramatic flat end.
Check that on the triangle whisk that the tines go into the handle and enclosed. You don't want there to be any way food or liquid can get into the handle.
Triangle whisks are typically made of stainless steel, but they can also be found in silicone.
These whisks are made full size (like the ones you see on this page) but they also are a popular type of cocktail whisk. These smaller longer versions of this whisk are designed to fit into a slim cocktail glass, not stir gravy.
It will typically be quite easy to tell which you are getting but it is worth mentioning.