For cooking tarts and tartlets, you can use classic moulds or tins (left photo), or rings (right photo) which are moulds with no base that are placed on a non-stick baking sheet for cooking. This is what pastry chefs use, because with no base, tart cook faster and more evenly, and it's easier to remove them from the ring.
If you intend to buy:
Use tins (better heat diffusion), preferably non-stick (easier to turn out). Rings should be stainless steel.