All three of the above replies are correct. Wet on wet refers to painting over a wet layer underneath, regardless of the type of paint you are using. Watercolors, oils and acrylics or about any type of paint can all be used with this method.
It is easily done with oils because of the long drying time of the medium.
Wet on wet watercolors are fairly easily done by wetting the paper before applying color. Lots of beautiful effects can be achieved this way.
Acrylics are very difficult to keep wet enough to do the blending, but it is possible, especially with additives to extend the open time.
Perhaps I should do an article on this! Thanks for the idea!