Is there a laymen FAQ out there about the different measurements one encounters and what they signify? It's pretty obvious when you look at a sea level pressure forecast, but the 200mb, 500mb, 800mb etc. are a little fuzzy to me. And what are the norms? Often you hear about the 500mb height line. Is this a point in the atmosphere where measurements are derived from or do they base interpretations off of seasonal norms or whatever?

Thanks for any tips or info.

Steve