An online slot is as it says, online and live whereas an offline slot is more of the old fashioned land based traditional casino type mechanical slot where pulling a level makes the reels spin.
The random number generator is often deemed as the brains of the slot machine.
Online slots, especially those that offer 500 free spins now run on RNG, random number generators. It is a device that generates a sequence of numbers or symbols that can only be predicted by random chance and not by any other means such as a strategy or trying to work out the game. There is no working out, it is purely random. The random number generator is often deemed as the brains of the slot machine. Most players do appreciate that there is a computer chip inside of the slot machine responsible for picking numbers but not everyone understands how it works and this can lead to misconceptions of the RNG. Some players believe that the RNG is similar to that of a progressive jackpot slot game and therefore runs on a cycle and eventually will hit and pay out. The RNG generates random numbers regardless of whether the machine is being played or not and it keeps repeating the process several billions of hundreds times per second. A dice, for example, is a manual random number generator, the dice is rolled and a random number from one to six appears. Rolling the dice in a particular way will not change the outcome of the number it gives.
Both online and offline slots can be progressive jackpots, in other words where the more money fed into the machine goes into a pot ready for the big win where the takings will be paid out, less the house edge, of course.
Both slots also work on RTP’s, return to player, percentages, online usually paying out a higher percentage than an offline slot. This percentage indicates to the player the odds of winning while playing with a set calculated house edge. The RTP percentage is calculated over millions of spins, not just from one specific player but from other players at the casino or online globally. Therefore, the winnings won’t be produced every time a player plays the game, however, eventually the player should win, in theory. The RTP percentage is theoretical and like the random number generator is exactly random. Slot game developers must advise players what the RTP percentage is and some casinos may group the highest Return to Players slots together. The only way a player can work this to a win despite the randomness of it all is to focus on the hit variable and the volatility of the game. For the beginner this is probably way too strategic but for the seasoned player it can work. A slot game with a high RTP percentage will have added features such as wilds, scatters, free spins and multipliers to not only make the game more exciting but to help obtain the RTP percentage.
Don't be fooled, an offline slot can pay out way more than an online slot. An anonymous software engineer won almost $40million on an offline slot in Vegas.