What is Hubble's Law and how does it relate to Hubble's constant?
Hubble's law has to do with the expansion of the universe. Space is expanding, every portion of it receding from every other, and the more distant two objects are, the faster they recede from each other. The rate of recession can be measured by spectroscopy (items receding from us show a light spectrum shifted towards the red end). This is Hubble's law: distant objects are receding from us, and the further they are, the faster the recession.
Hubble's constant is the rate of recession per unit distance. This can be used as a distance measure. I.e., capture the light spectrum from a distant object (generally a galaxy), measure the amount this has shifted towards the red end of the spectrum, and based on that you know how far the object is from us.