Personally i always use a HB pencil (a Staedtler one natch) but what does HB actually mean? This code refers to the hardness of the pencil graphite. The grading (in Europe at least, the US and elsewhere use different grade classification schemes) runs from at least 9B - which are the softest pencils - useful for artwork shading - through to 9H or beyond - these are so hard they can puncture holes in plate armour. B stands for Blackness and H for Hardness. There is also an F which is between HB and H in hardness, the F refers to the fact the graphite can be sharpened to a fine point.
This code has been used since the early 19th century. However, there is no standard for pencil hardness. HB or 2H from one manufacturer might be slightly different to another manufacturer. I find the Staedtler HB suits my purposes perfectly, as you would expect from a grade which is right in the middle!
