Watercolor paper is also sold in blocks that are 'stuck together' at the edges. This format has the advantage that the paper doesn't have to be stretched before you paint on it to avoid it buckling.
There are disadvantages to watercolor block though. For starters, you have to leave the painting to dry in the block (if you separate a sheet off before it's dry, it may buckle as it dries). Which means that you need more than one block if you want to do several paintings on after another.
Also, some manufacturers don't assemble their blocks so that the same side of the paper is always at the top. So you may find yourself painting on the 'right' and then the 'wrong' side of a paper. And I've heard artists say paper in a block didn't have the identical surface texture as the same brand in a single sheet, so watch out for that.
Watercolor paper sold in blocks is usually more expensive than any other format, but the convenience may make you decide it's worth it.
