I would like it if you could use it with elastic and inelastic collisions. When is momentum lost or gained in either collision, when is energy gained in either collision. What IS conservation of momentum and what IS conservation of energy

(may be broke/outdated!)
I would like it if you could use it with elastic and inelastic collisions. When is momentum lost or gained in either collision, when is energy gained in either collision. What IS conservation of momentum and what IS conservation of energy