Symbols are the term used when talking about the images shown on the reels. There are traditional symbols in many slot games. Fruits of all varieties, bar symbols, high-card numbers from a deck of playing cards. Each one has its own value and each one has its own history behind it.
The original slot machines used lucky symbols that became common in the 1900s. There were bells, and diamonds, hearts, horseshoes, spades and the high-card deck symbols like King and Queen, and Jack. Later a Bar symbol was introduced to indicate a win. The slots were usually found in bars and pubs commonly so in order to get a winning prize you had to go to the bar to collect some form of winning prize. Perhaps a drink or bartered amount of money.
The symbols today are very different than ones from the old days. Slot games today can provide exciting animations within the symbols. They are all part of the show that makes the slot games more interesting and lively. New versions of the 3D slot games have shockingly realistic symbols that do something every time certain combinations are landed. Music and sound effects add to the bells and whistles found in older games but it's really the same concept.
Every symbol can form a combination that is worth something. Depending on the symbol, amounts of money, free spins, or bonus games can be awarded within the game itself. The more symbols present means that the lower the odds of winning become. This doesn't mean that the more symbols are always a bad thing; it just might enhance an otherwise boring game.