There are two systems for grading lead hardness: European and American. The European system uses a combination of letters and numbers. B denotes soft leads, or leads with greater graphite content. The higher the corresponding number, the softer the lead and the darker the marks produced by the lead. At the other end of the spectrum, H designates leads with higher clay content. H leads become lighter as you work up the scale.
The American system uses numbers, with #2 ½ in the middle of the scale. The American #2 pencil usually lines up with the European HB grade, as seen in the graphic above. There is no industry standard for the hardness of a lead grade and results vary from brand to brand. Japanese leads tend to be darker than their European equivalents, though they use the same system. We reference the European grading system for the remainder of the article as it is more comprehensive than the American system.
I found this, and other interesting information here.
I like 2mm 2H lead because I am a leftie and it does not smudge like the more common HB. I think though, looking at this chart, that H may be a better fit for me. What is your favourite lead hardness?