If the housing and lens are still in good condition and the vehicle uses a replaceable bulb, you may only need the correct Headlight & Tail Light Bulb.
If the lens or housing is damaged, mounting tabs are broken, moisture is getting inside, or the light uses an integrated LED that isn't separately serviceable, you may need the complete Headlights & Assemblies or Tail Lights & Assemblies unit instead. Replacement assemblies can include different combinations of housing, lens, reflector, bulbs, sockets, or wiring, so check exactly what's included.
If the light itself is fine and you're replacing a bracket, harness, socket, lens, or another small piece, start with Headlight & Tail Light Components instead of buying the whole assembly.
This should be the centerpiece FAQ for the parent.