Collisions between objects in an isolated system always conserve linear momentum, however the same cannot necessarily be said about kinetic energy.

Conservation of kinetic energy is dependent on the type of collision we observe.

Elastic Collisions

In interactions involving only conservative forces, energy is conserved. For a collision, this means that the total kinetic energy of the system is the same before and after the collision. Deformations during the collision temporarily store energy like a spring before returning all the energy to the interacting objects in the form of kinetic energy. We call these types of collisions elastic.

Inelastic Collisions

For interactions involving non-conservative forces, like friction, energy is transformed into other forms during the interaction (ex. heat, sound, permanent deformation, ...) and is lost to the environment. For a collision, this means the total kinetic energy of the system will be lower after the collision than it was before. We call these types of collisions inelastic.