For the most part, online slots work the same way as the majority of slot machines that you will find in casinos around the world. You can access online slots from your desktop computer or mobile device.
Just like Vegas slot machines, online slots use an RNG to generate results for each spin. This means that each spin is as close to truly random as possible.
Unlike a deck of cards in an actual, physical casino, the exact time that you hit the spin button will play a role in determining what the result is. If a deck of playing cards is set, the cards will not change based on your decisions. With slots, though unpredictable, your actions do impact the result.
In terms of win frequency or games offered by betting limits, online casinos tend to offer lower limits than most land-based casinos. This is for a few reasons. First, online slots allow for a much faster pace of play. If the online games are 2x faster, you will effectively be risking the same amount if you play for $0.50 cents per spin online, as you would be risking $1 per spin in your local casino.
The casinos know this, and lower limits help to encourage more play. While some online casinos offer high limit slot play, the majority do not offer many games over $20 or $30 per spin.
Another difference is that it can be harder to find slot tournaments online.