raisins, sultanas, and currants all terms meaning a dried grape. Raisin can be used in a general sense, to apply to all such items, but is usually taken to mean a dried grape which is not a sultana or a currant. Sultana is sometimes a noun, sometimes a variety name (Sultana grapes), and sometimes an adjective (sultana raisins). Currant is a plain noun.
Raisins of most kinds originally came from the Mediterranean region, the Middle East (especially Turkey), and W. Asia (notably Afghanistan). They are still produced in these regions, but the leading producer is now California, where the main raisin varieties of grape are the following:
California White Muscat. The grapes are amber to yellow-green in colour, thin skinned with firm, rich, moderately juicy, finely flavoured flesh. Grown especially for raisins as it is too tender to be shipped fresh.
Thompson Seedless (or Oval Kishmish or Sultanina). The grapes are yellow with low acidity and entirely seedless.
Sultana (or Round Kishmish) is grown in limited quantities. The grapes have a higher acid content than Thompson Seedless and occasionally develop seeds.