European merchants may be required to charge VAT, depending on their and each buyer's country; this is determined automatically when you enable VAT for your account and products.
In the USA, sales tax rates and requirements vary by state, county and city, so you'd want to contact your local Dept. of Revenue or equivalent for info about your local laws. Most typically, merchants are required to collect at least their own state's sales tax rate for buyers who are located in the same state; this may vary according to product type (tangible vs. digital vs. nontangible services), and your county and/or city may also require you to collect tax according to their rate for buyers sharing your location as well.
