Dobro is a brand name of resophonic guitar. I believe the name is currently owned by Gibson. If a guitar is not made by Dobro (Gibson), it is not a Dobro. Single resonator guitars come in biscuit and spider bridges. Biscuit bridges are most often used for slide or blues playing while spider bridges are often used for lap style bluegrass playing. There are Dobros with biscuit bridges and spider bridges and there are non Dobros with biscuit and spider bridges. There are also wooden bodied National guitars with spider bridges.
There are also other types of resophonic guitars, some with three resonators.
I have also seen fake resonator guitars with a cover plate fastened to a top painted with a silver circle.