you got a reply with a link to more information, but there is plenty
more out there. Fine Woodworking ran a decent in depth article in the
last year or so that covered the three grit sizing systems pretty well.
IIRC, one is japanese, one european and the other north american.

steel wool works differently than sandpaper, so there's not a direct
correlation between the numbering systems other than that they indicate
progressively finer products.