The classic machines of slots and their modern version are different in how they work. First slots machines were mechanical devices. Players activated the machine by pulling a lever handle. The reels would then spin and if they stopped in the right combination across the 3 reels the machine would dispense your winnings at an opening near the bottom of the machine.
Modern slot machines are much more sophisticated. The machines actions are run by computers. The jackpots, sound effects, and the reels are controlled by the computers. The computer generates random numbers when you activate the slot. The computer runs a formula that is mapped to the stops to show you either a winning combination or a losing one.
While you might not be able to play a classic slot machine at an online casinos many still offer that style of game play powered by the modern slot technology.