You can set up hair to collide with itself, the ground plane or other objects. You can even use the Collision Sphere and Collision Cube constraints to establish implicit collisions. For more information, see in the Hair guide.
Use hair constraints to hold hair in position for a hairstyle, such as a ponytail or a clasp. You can even use hair constraints for more realistic hair behavior, such as using the Transform constraint to hold the hair to the head. For more information, see in theHair guide.