The explanation looks a bit complicated, but here it is:
Sultanas: seedless, large-berried and light yellow. Larger than currants and smaller than raisins.
Currants: seedless, small-berried, purple/black colour. Their name derives from the Greek city of Corinth.
Raisins on the bunch: seeded, large-berried, generally with stalk.
and to expand on Wolfgang's answer, a Sultana is a dried grape of the Sultana variety, which is a white seedless grape, thus the dried sultana is softer than other dried grapes as it has no seeds inside. I prefer it when making scones