There's an interesting explanation of slot machine operation about halfway down the page at Las Vegas gaming tips site http://www.vegasslotsonline.com/work/
How do Slot Machines Work?
Slot machines are pretty simple. In a three-reel game, a random number generator (RNG) picks three random numbers, each of which corresponds to a stop on each reel, then machine spins the reels so that they stop on the spots selected by the RNG. The principle is the same for five-line video slots: five random numbers, one for each reel.
Note that by the time the reels are spinning, the game is already over. The RNG has already selected the stops, and the reels spin sort of as a courtesy to the player. Slot machines don't even need reels -- you could just put your money in and the machine could tell you whether you won or lost. The presence of the reels makes no difference in the game.
The reels are weighted so that some symbols are more likely to hit than others. For example, the slot may pick a random number from 1-127, which might correspond to the symbols as in this hypothetical example:
1-66: Blank (positions between reels)
67-77: Single Bar
78-85: Double Bar
126-127: Jackpot Symbol
Say the computer picks #53. That's a blank, and it instructs the reel to stop on a blank. If it picks #82, then it tells the reel to top on a double bar. Most of the numbers are for the lower-paying symbols, so that's what's more likely to get chosen.
NB this only applies to the more recent electronic machines, the mechanical ones relied on a braking system to slow the reels down and caught in the ratchet at a certain speed to stop each one. http://entertainment.howstuffworks.com/slot-machine.htm