Pencil Lead Hardness (from Wikipedia)
Pencil lead is graded by hardness: a greater percentage of clay results in a harder lead. H (harder), F, HB and B (softer) are pencils of average hardness. Pencils ranging from 2B to 9B are softer still and are used for sketching; 2H to 9H are harder than average. A common American #2 pencil is equivalent to HB.
In order, the leads are (from hardest to softest) 9H | 8H | 7H | 6H | 5H | 4H | 3H | 2H | H | F | HB | B | 2B | 3B | 4B | 5B | 6B | 7B | 8B | 9B.
The harder the pencil the lighter the mark, e.g. Bs tend to a create darker colour that can be smeared easily.