What is the difference between Play-Doh, plasticine, air-dry clay, and polymer clay?

Play-Doh and similar dough compounds are soft, water-based, and reusable but dry out if left uncovered. Plasticine (oil-based modeling clay like Crayola's bold-color sticks) never hardens and is good for repeated reshaping. Air-dry clay hardens on its own over a day or two without baking, making it good for keepsakes. Polymer clay is oven-baked to a permanent hard finish and is generally aimed at older kids and crafters.