A classic slot is a game styled after traditional slot machines. Most have three reels, a small number of paylines, and symbols such as cherries, lemons, bars, bells and sevens.
Features are usually simple or absent: perhaps a wild, a multiplier or a respin, rather than elaborate bonus rounds. That makes the game easy to follow, but it does not mean the game is lower risk. Some classic slots are quite volatile, because a big part of the return sits in a few top combinations.
Online, the classic style is a design choice; the games use an RNG and are certified like any other. The look traces back to the Liberty Bell and the British fruit machine.