Pencil Lead types are graded by hardness and by how dark a mark they leave on the paper. Generally, the harder the pencil, the longer it lasts, but the less lead it leaves behind. (obviously.) Softer pencils leave heavy, dark marks. Standing right in the middle of the pencil hierarchy is the infamous #2 pencil, known to the rest of the world as "HB". Leave it to America to develop a different scale than the rest of the world uses.. There is a large assortment of lead types available, although most of the extreme types are probably only used by artists.
The scale runs from 9B (the softest) to 9H, with a few funny named ones in the middle:

9B 7B 6B 5B 4B 3B 2B B HB F H 2H 3H 4H 5H 6H 7H 8H 9H
This scale runs from Softest (left) to Hardest (right)

The American scale, although somewhat deprecated, is equivalent to the following.
#1 - B
#2 - HB
#2.5 - F
#3 - H
#4 - 2H
These comprise most daily use and writing pencils.

All wood-encased pencil leads (I don't know about mechanical pencil lead composition; it may differ) are comprised of three major ingredients - Graphite, Clay, and a small amount of wax. (The amount of wax generally remains consistent and small regardless of the pencil type) The more graphite the lead contains, the softer (and blacker) the rating.

Pencil Lead Compositions
Pencil Number Graphite Clay Wax
9H 41% 53% 5%
8H 44% 50% 5%
7H 47% 47% 5%
6H 50% 45% 5%
5H 52% 42% 5%
4H 55% 39% 5%
3H 58% 36% 5%
2H 60% 34% 5%
H 63% 31% 5%
F 66% 28% 5%
HB 68% 26% 5%
B 71% 23% 5%
2B 74% 20% 5%
3B 76% 18% 5%
4B 79% 15% 5%
5B 82% 12% 5%
6B 84% 10% 5%
7B 87% 07% 5%
8B 90% 04% 5%

Edit: Thanks to arcanehl for confirming and informing me that 9B pencils were available. If anyone finds the composition of 9B lead, which I imagine would be somewhere in the ballpark of 93% graphite, I'd be glad to post it here.

Bibliography Information:
Composition Information from Computer Graphics Forum, Volume 19 (2000), Number 1 pp. 27-49 "Observational Models of Graphite Pencil Materials", Mario Cousta Sousa & John W. Buchanan. Available at http://pages.cpsc.ucalgary.ca/~mario/npr/projects/PencilMaterials/Sousa-Buchanan-cgf00.pdf

Hardness Scale information from Staedtler Product Information on Wood-cased pencils, available at http://62.52.109.14/e4wstaedtler/central/resource.nsf/imgref/F18A8B437146D4B5C1256B9F003C3559
/$FILE/Holzgefasste_Bleistifte_2002_english.pdf

American Hardness Scale equivalents from "Pencil" entry in the Wikipedia, located at http://en.wikipedia.org/wiki/Pencil

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