A tug or more commonly a tugboat is a secondary boat which helps in mooring or berthing operation of a ship by either towing or pushing a vessel towards the port.
A tug is a special class of boat without which mega-ships cannot get into a port. Along with the primary purpose of towing the vessel towards the harbour, tug boats can be engaged in the purpose of providing essentials such as water, air, etc. to the vessel.
Tug boat eases the manoeuvring operation of vessels by forcing or tugging them towards the port. Mega vessels can never be manoeuvred by their own. Also with the increased size of the boat, they need tug boats to carry some of their domains and tow them through narrow water channels.
Also tug boats become essential elements for non-self-propelled barges, oil platforms, log rafts etc.