Resin coated, or RC, papers have an ultra thin plastic coating on both sides of the paper. FB papers, since they have no protective coating soak up considerable chemicals and water during processing. Therefore, they require very long wash times to purge the paper of its contaminants. RC paper don't have this problem and require short washing periods before drying. FB paper require more work than RC papers, yet they are still considered the standard for exhibition, museum, and archival prints.
Contrast refers to the paper's difference between pure black and pure white on the exposed paper.
A very low contrast paper, grade 0 for example, will not print a deep black or a bright white, only a number of shades of gray. A very high contrast paper, grade 6, print only a few shades of gray between the extremes of a very dark black and a brilliant, blinding white. Papers come in two styles of grade classification. Using graded papers means you must buy each paper in each grade you are going to use. Variable contrast papers use separate filters to achieve a contrast range from 0 - 6.