Resonators are a type of a acoustic guitar which can be identified by a large metal plate on the top or soundboard of the body. Whilst a conventional acoustic guitar is amplified by the soundboard of the guitar when playing, a resonator style acoustic instead amplifies it's sound through the large metal section which a resonator is named after. This resonator is normally made of one, three or more spun metal cones depending on their build style, which vibrate and make a much louder sound than on a normal acoustic.
Dobros are a the name of a popular style of resonator acoustic guitars and are named after the original manufacturer of this style. These have one metal coil which is inverted and attached to a spider style bridge. These are more commonly made with a square neck, which are designed for playing a lap steel style of music that is popular in bluegrass and country and often made with a wooden body. Other styles of resonator design such as biscuit and tricone are often made with metal bodies and a rounded neck, making them ideal for playing like a conventional acoustic in blues music. There are however many combinations of neck shape, resonator style and body material used in various blues, country and folk music genres.