Caltrop Grenades scatter sharp spikes which damage enemies who cross over them, unless they move slowly. The caltrops will not hurt teammates, and will remain in the world for the length of the Ability's cooldown period.
Caltrops are useful for support in large firefights, and are generally best placed where enemies will want to fight or maneuver, as the damage from simply running through them will rarely be significant. However, they will grant you a decent amount of XP whenever enemy forces take damage from them.
They can also be useful in discovering enemy positions, as the player will be notified when an enemy treads over the caltrops through the experience gain popup. However, this can be circumvented if the enemy is crouching or walking through them while aiming down the sights.
Caltrop Grenades are also useful for cutting off narrow passages or shortcuts, as enemy forces must either go around, or drastically slow their pace while moving, allowing you to easily line up a nice headshot or two.
Objective defense is another good use of the Caltrops Grenade.