Most of the best sketchbook brands will display the grams (or lb) per square metre of their paper. This refers to the weight of the paper. Generally, the heavier, the higher the quality of the paper.
Paper with a weight of 75-90 gsm / 50-60 lb is generally heavy enough for sketching with pencils, pastels or charcoal but too thin for ink or markers, which could bleed through it. A weight of around 100-130 gsm / 70-80 lb is generally suitable for finished pieces in a range of media. For very rough sketches, any kind of paper can suffice, even office printer paper, which tends to have a weight of around 80 gsm. However, it doesn't offer the most satisfying feel and isn't the most practical to carry around and keep your sketches.
For watercolours, you will generally want a paper of at least 300 gsm / 140 lb. Ideally, you will want heavier paper than that to avoid having to stretch the paper beforehand. Cotton paper, sometimes called 'rag paper' is better than wood pulp paper for watercolours, especially if you do a lot of scrubbing and masking, but it is usually more expensive.