Generically, a "truck" could be almost any vehicle used by the fire department, but the term has become specialized over the years.
Originally, "engine" referred exclusively to "pump", the important tool for getting water to a fire. Today, "fire engines" are those vehicles of the fire department that pump water. The term "truck" is reserved for other types of vehicles, usually having one or more ladders.
Since Fire Engines and Fire Trucks perform significantly different functions at a fire scene, they are very different. Fire engines are equipped with hoses and water so that personnel can aggressively fight the fire. Fire Trucks are like the firefighter's tool box -- carrying ladders, rescue equipment and other tools to enable personnel to support firefighting activities.