2b - the B indicates softness of the "lead" - so a 4B is even softer than a 2 B
2H - the H indicates the hardness of the "lead" - so a 4H is harder than a 2H
Hard pencils generally draw lighter, but hold a fine point longer. Soft pencils draw darker, but lose a point very quickly, and the lines get wider.
HB is in the middle between hard and soft.
Technical drawings - like an architectural drawing, you need a fine point, so use H pencils.

Drawings - top one is pretty good - the best of them, I think you need to move the right nostril up and in a little. Oh, and darker in the nostrils - it's dark in there! The next two, the horses' necks are too narrow, wouldn't hold up that heavy head well. The neck on the last one is much more appropriate.