In the early days of pool, pool cues were used with no tips on them. The leather cue tip, with which a player can apply side-spin to the ball, was perfected by the year 1823. Players from England showed Americans how to use spin, which explains why it is called "English" in the United States but nowhere else. The British refer to it as side.
The cue tip is made of leather of varying degrees of hardness. Some cue tips are made of pig skin, cow leather and even buffalo hide. There are cue tips on the market that are made of a mixture of leather and synthetic materials which are designed to last longer than leather tips.
Cue tips also come in various sizes, from about 11mm to 14mm, and colors. There are also layered laminated tips available. Each layer has a different property to give the cue tip its unique hit characteristics. The harder layers are placed at the bottom of the cue tip for accuracy. The softer layers are placed at the top of the cue tip for gripping the cue ball during the shot. This allows the player to not only get used to the tip, but not have to change them as often as non laminated tips.
Some cue tips are designed for specific shots such as jumping or breaking. Cue tips can be glued onto the cue and some can be screwed on or slipped on. Cue tips are usually purchased in boxes and can cost as low as $10 for a box up to $20 for a single tip. Moori and Sterling are two well known cue tip companies. There are also accessories available for your cue tips such as glue/cement, cue tip sanders, and home tip repair kits and cue tip clamps to help you change your tips.