Have you ever wondered what the letter and number on the end of pencils stands for?
It's actually a way to describe the ratio of clay to graphite in the lead, which determines how hard/light or soft/dark it will write. H stands for hard, and B stands for black. HB stands for hard black and is typically at the middle of the spectrum, with F being the closest to it and having just a little bit more clay in it than HB, which makes it a bit firmer and lighter. 
Artists will typically use a range of the pencils, using the softer and darker ones for shading and blending and then the harder ones for crosshatching or fine line work. For writing, people typically stay between 2B and HB, but a lot of companies have branched out and created a larger range of ratios in all sizes of lead as people shift to using more mechanical pencils for drafting and drawing.
