Cover is heavy weight used for booklet covers, folders and cards. It can have any surface texture and often is made to match a line of quality text and envelopes. Duplex cover is two sheets of quality text laminated together, usually two colors. It can be quite good for artwork and book arts because of the stiffness and the design options offered by the two colors. Sized means there is some gelatin or polymer applied to the paper surface or into the pulp to make the paper stronger and less likely to soak up moisture.
Pulp is the material paper is made from.
Wood chips are cooked in acid to break up the fibers. Residual acid can react with things in the air and make paper weak over time. Plenty of water is needed to clean the pulp enough to make good paper. Ph refers to the degree of acidity or alkalinity in paper pulp. Most machine made paper is acidic, and mills now put things like calcium carbonate in to counteract this and to allow a Ph neutral label.