Pencils are sold in various levels of hardness, from very hard to very soft. The hardness of a pencil is usually indicated on it in small letters towards the top. Hard pencils are indicated with a H (which stands for 'hardness') and soft pencils with a B (which stands for 'blackness') plus a number from two to nine to indicate the degree of hardness or softness. Most pencils sold in stationery shops are HB, neither hard nor soft.
Hardest to softest: 9H, 8H, 7H, 6H, 5H, 4H, 3H, 2H, H, HB, B, 2B, 3B, 4B, 5B, 6B, 7B, 8B, 9B.
The graphite (or "lead") in pencils is the same, a mixture of graphite and clay. The degree of hardness is determined by the temperature at which this is baked.