Well, they are similar to other kinds of slots. They have a set number of reels, rows, and paylines or ways to win, while thematically, these slots have much in common.
Most of the time, this is a fruity theme with heaps of fruit symbols, lucky 7s, golden bells, and the rest of the classic symbols crew.
Classic online slots use a random number generator to choose which reels and symbols to show on the screen. Naturally, as the technology progressed, the number of reels, rows, and paylines increased, offering bigger payouts on newer classic slots.
The old machines with physical reels could only have a limited number of combinations because each reel could only hold a certain number of symbols. Many people who liked the old classic slot machines wanted the same look and feel of the game but also wanted the chance to win bigger jackpots.