This is another informative site:
http://sewing.about.com/od/sewingmac...Sizes.--94.htm
Sewing machine needles:
The larger the number, the thicker the shaft of the needle.
For piecing ordinary quilting cottons, I use a 70 or 80 Schmetz needle, so far, I haven't noticed a major difference if I'm using a ballpoint, universal, or regular needle.
Hand sewing needles:
The larger the number, the needle is slimmer and shorter (for a given style/type of needle)