Watercolor paper
Watercolor paper is paper that is made to take a lot of water and can be made from either wood pulp, cotton, or a combination of both.
Top grade watercolor paper is 100% cotton and acid-free, but you don't need the absolute best to make beautiful art.
Hot vs. cold-pressed
Hot-pressed watercolor paper is smoother in texture and is easier to stamp on. Cold-pressed watercolor paper is a little bit rougher. If you want to stamp on this type of paper, which you absolutely can, a stamp positioner comes in handy so you can restamp the bits that didn't transfer well.
Block, spiral bound or sheets
Some watercolor paper is sold in loose sheets of different sizes. Keep in mind that if you want to tape down your paper on all sides onto a surface (to prevent your paper from buckling), you should take a paper size that is slightly bigger than what you want your end result to be.