Scientifically all taods are frogs. There is no scientific category for 'toads'. However commonly when people talk about toads they are thinking about members of the family bufonidae (like bufo bufo, the common toad). Toads generally have warty and dry skin and usually prefer dryer conditions whereas frogs have smooth and slimy skin, and usually prefer more humid conditions. However there are exceptions, for example the fire-bellied toad, a very popular pet, is aquatic. go figure. Frogs tend to lay eggs in clusters while toads tend to lay long chains. Toads have paratoid poison glands behind their eyes, but then so does the giant waxy monkey frog and it is considered a frog.
As I said there is no scientific definition of 'toad', they are all anurans (tailless amphibians), the toad is a lay concept and hence is quite flexible.