The United States does not have one rule for slot machines. Each state decides whether they may be offered, where, and under whose license. What was true of riverboats or racetracks can be amended, so treat this page as a map of the ideas, not as legal advice for a trip or a business.
Where the machines have been allowed
Nevada has long had the most open market. That is why a visitor still associates Las Vegas with slots in casinos, and historically in a wide range of businesses. Other states were narrower. Illinois, Indiana, Iowa, and Missouri built much of their casino industry on licensed riverboats or docked barges. After Hurricane Katrina, Mississippi let Gulf Coast casinos move ashore rather than stay on the water. Delaware’s machines at horse tracks have been tied to the state lottery. These are patterns, not a complete list, and several states have added commercial casinos since the riverboat era.
Tribal gaming, and two classes people mix up
Casinos on tribal land follow federal Indian gaming law as well as a compact with the state. The compact is the pact that sets what may be offered and how revenue is shared. In that federal vocabulary, Class III is the traditional casino game: a house-banked slot, among other things, where you play against the house and each spin stands on its own. Class II is bingo and electronic games built on bingo. It is not a nickname for a video lottery terminal.
A video lottery terminal, or VLT, is a different product. States such as Delaware have used centralized lottery systems, with a host computer overseeing terminals and, in some designs, players competing for a common result. People fold that idea into “Class II” because both can look like a slot screen. The legal categories are not the same, and the player is not always “against the house” in the Class III sense.
Slot clubs
A slot club is the casino’s loyalty book. Insert the card and the floor records your play, then returns a fraction as comps: a meal, a drink, a room, sometimes cash back. No card, no record. Online casinos get the same ledger by requiring a sign-in. The program matters on a cash floor because the comps are real goods. It has nothing to track when the credits were never money.
Slothill is not a U.S. casino and does not offer real-money slots in any state. It is free browser play. For the separate question of what an online casino site even is, see online casino types.