From A Wiki of Ice and Fire
An arakh is a sword commonly used by the Dothraki. It is shaped half like a sword and half like a scythe.  There is at least one known Valyrian steel arakh in the world, it is currently wielded by Caggo. 
According to a representative of Valyrian Steel  HBO arakhs are similar to khopeshes whereas in A Song of Ice and Fire, arakhs are more like scimitars. The arakh has a very significant curve, similar to very fine Persian shamshir swords: 
See Also: Armament, for further info on various weapon types.