The words are literal , a water tube boiler is water inside the tubes with hot gases surrounding them and a fire tube boiler are got gases within the tubes with water surrounding them.

Fire tube and water tube. Steam Boiler and Water Boiler.

the advantage is the design is complicated

The answer is in the name. Boilers are made with a "tube - in - shell" construction, and the name implies what is in the tube. In a water tube boiler, the water circulates through the tubes, and the burner is usually under them. Most of the time, the "shell" of this style of boiler would not be watertight. A fire tube boiler has a watertight outer jacket with tubes through the center of it. The flame is shot through the tubes.

there are two types of boilers water tube and fire tube. fire tube boilers are divided in to two categories vertical and horizontal. this according to the construction of fire tubes. it is said that vertical boilers are inefficient than horizontal boilers.

The vertical boiler is small capacity, and horizontal boiler is higher capacity, well, the horizontal boiler have many types according the drums, there are single drums boiler, and double drums boiler, while, the single drums boiler have fire tube boiler and water and fire tube boiler, double drums boiler is usually water tube boiler.