A tug is a small, strong watercraft used to pull or push barges or assist larger boats in maneuvering. It is a maritime vessel that pushes or pulls other boats using direct contact or a towing line. Ships that can't move on their own, like those in busy harbors or tight canals, or those that can't move at all, are usually towed by these boats.
A barge is a large flat-bottomed, towed, self-propelled watercraft used chiefly for transporting heavy items or bulk freight along rivers and canals. It's a coastal commerce ship with a vast spritsail and jib-headed headsail, a fore staysail, a short mizen, and leeboards instead of a keel.