Maybe this will help:
Lace weight is very, very fine, sometimes called "cobweb", which gives you an idea. It's hard to define how many stitches per inch you get, because you usually aren't using a regular stockinette stitch with it, but instead, these open, lacey-type stitches.
Sock or fingering weight gives you about 7 - 8 stitches per inch, and is, as it sounds, used for socks and gloves.
Sport weight is usually about 5 - 6 stitches per inch.
WORSTED WEIGHT is usually about 4 -5 stitches per inch. If you can get to something like Joann's or Michael's or Hobby Lobby or AC Moore, look at Vanna White's yarn, or Red Heart Classic or Lion Brand Wool-Ease. No 2 worsted weight yarns are exactly the same thickness, but those are pretty standard examples of worsted weight.
Aran weight is slightly thicker, and used for Irish fisherman knit sweaters-- the ones with all the cables and bobbles and often in white or cream.
Bulky weight is quite thick, and you get maybe 2 - 3 stitches per inch. Lion Brand Thick 'n' Quick is that kind of thing.
And then finally you get into novelty and super-bulky yarns.
Hope this helps!