What paper do you use for watercolor painting?

Watercolor paper needs a minimum weight of 200 g/m² or it'll warp when you wet it. There are three main categories: cold-pressed (rough), which has a pronounced texture and holds more water; hot-pressed, which is smooth and good for details; and satin finish (or cold-pressed satin), which is the most versatile compromise. Most beginners start with the satin finish because it works decently for pretty much everything.

Then there's the composition: 100% cotton is the best because it absorbs and releases water in a controlled way, and lets you lift and correct color even when it's dry. Pure cellulose paper is cheaper but less forgiving - the color sets immediately and mistakes just stay there. Then there are the blends.

Well-known brands are Saunders Waterford, Arches, Schmincke, Hahnemühle. If you're just getting started, try a pad of 200-300 g cotton/cellulose mix - it costs less and you'll figure out if this is actually a hobby you'll stick with. Once you know what you're doing you can invest in better paper.