Swimming caps should be worn by anyone with long hair or competitive swimmers wishing to make faster times. Watch any Olympic swimming race and you'll notice that every racer wears one. Many pools these days also require use of a swim cap.
Invented in the early 1900s, the first swim cap was made out of rubber. In the 1920s, latex caps began appearing. Later on, silicone versions hit the market. A number of swimwear companies manufacture swim caps, including Speedo, Nike, TYR, and Dolphin. Most typically cost between $5-$10. Silicone tends to be the most expensive material but the most durable, while latex is the cheapest. A good swim cap will keep your hair dry and free of chemicals like chlorine as well as reduce drag.
Here are some general tips for using and taking care of a swim cap:
Silicone swimming caps are the most popular. They never loose elasticity and last a long time. Latex is also a popular choice. Insulating caps made out of neoprene are commonly worn for triathlons or for swimming in cold water.
Caps can be found at any sports retailer that sells swimwear. From my experience, however, the best prices can be found online. Some like to buy locally so they can make sure they get the right fit, but if you have a normal-sized head this shouldn't be a problem. Swim caps are one size fits all for adults. Smaller kids versions are also sold.
If you are a competitive swimmer, talk to your coach about wearing a swimming cap. He/she might have recommendations, or everyone on your team might wear the same swim cap.
Whether you're swimming for the first time or a seasoned veteran, I hope you enjoy your stay on this site. It contains a wealth of information- more than any other similar resource I've seen.
As always, exercise safety in the pool. Swimming is a fun hobby I've enjoyed for years, but anyone who's been around a pool for more than a few years has witnessed their share of accidents. Personal safety should be your top priority.