Worsted-weight yarn is slightly finer than the UK Aran-weight yarn working up at approximately 16-20 stitches per 10cm on 4.5 - 5.5 mm needles.  Both Worsted- and Aran-weight are roughly equal to Australian/New Zealand 10ply.

"Worsted" also applies to a method of spinning, so if you're looking at handspun yarn you should check its weight.