H, HB and B describe how hard the lead of a pencil is. H H2 H3... H9 are rather hard mines (the higher the number the harder the lead), HB is a medium hard pencil (this is the one most common) and B, B2, B3... B8 are soft pencils. Though soft pencils are better to create shadow effects (because you can blur their lines by wiping over them) I frequently use hard pencils (H or H2) because you never have a problem with lines being smeared unintentionally.