Wrecks are... well, the wrecks that vehicles leave behind when they get destroyed, regardless of which type of vehicle they are.
Wrecks can be salvaged by Wehrmacht Pioneers for munitions, provide Heavy Cover, and block movement for light vehicles and infantry, although infantry rarely would need a long detour as they can move through really narrow paths.
Crushing them with a heavy vehicle provides XP toward Command Points, provided they were enemy vehicles destroyed in combat and not wrecks initially found in the map. Light vehicles with cannons and large caliber guns (such as artillery and AT) can also destroy wrecks if they're ordered to fire at the wreck. It isn't recommended to destroy a King Tiger wreck if you have a Panzer Elite ally, unless you're on the Allied side, where it's a good idea to destroy the King Tiger wreck to prevent recovery.
The Axis are also able to 'repair' wrecks with the Panzer Elite vehicle, the Bergetiger Repair and Recovery Vehicle, which also happens to be the fastest (and arguably strongest) repair units. Recovered wrecks usually have engine damage, and only have a portion of their health, so don't expect a recently recovered wreck to defend your Bergetiger. Note that recovering wrecks of vehicles that belonged to another player returns it to said player. Allied wrecks also can't be recovered to be used by the Panzer Elite.