|By Paweł Kuźniar, via Wikimedia Commons|
A bowl of raisins, including sultanas (and possibly currants)
It turns out that 'raisin' is an umbrella term, meaning that all currants and sultanas are raisins (but not all raisins are sultanas or currants). A raisin, then, as most of you are probably aware, is simply a dried grape. Currants and sultanas, then, are dried versions of grapes of a specific variety.
Currants are the dried incarnations of Black Corinth grapes, from Zante.
Sultanas, traditionally, were dried forms of the Turkish sultana grape. More recently, the name is applied to any raisin made from any variety of white grape, or any variety of red grape that is bleached to look white.