Collision where the total kinetic energy of the particles before and after the encounter is the same is called elastic collision.
Collision where the total kinetic energy of the particles before and after the encounter is not the same is called inelastic collision.
The danger caused by collisions is a result of the sudden high acceleration whic can damage equipment, cause injuries and even kill. It can cause damage from the deformation of object. It can cause colliding objects to shatter and fly apart at high speeds.
On the other hand collisions can hammer nails. They are used in sports such as pool, golf, and baseball. They have also been used to probe the makeup of matter by way of near light speed collisions inside particle accelerators.
In conclusion collisions are sudden interactions that can be elastic or inelastic. They can be dangerous and destructive but yet they can be useful.