Graphite is the dark gray material that is found in most pencils. Graphite is most commonly referred to as pencil or lead. Graphite is a grayish material that is available in a variety of different grades.
"HB" is most commonly called a #2 pencil and is found in the middle of the grades. Softer graphite pencils are named "B" pencils and range from "B" (soft) to "9B" (extremely soft). "B" pencils generally make darker marks because of the softness of the graphite. One downside to "B" pencils are that they become dull easily. "H" pencils are made of harder graphite and range from "H" (hard) to "9H" (hardest).
"H" pencils make lighter marks but keep a sharp point for a longer period of time. Graphite also comes in large chunks that are not in pencils. This graphite is used for covering large areas of a surface quickly.
*NOTE: The quality of the graphite pencil that you use for your drawings does affect the quality of the result. Check out the brands I recommend here.