Also a bicycle
built for two. The front rider is called the captain the rear rider is the stoker
A tandem allows two people of differing bicycling skills to stay together. A tandem is faster than a single; tandems go like a bat out of hell on downhills. They are also faster on the flats, but are slower going uphill. They are also more stable than singles, but this also makes them less responsive. When riding a tandem you can't make the kind of last second changes in direction that you can make on a single.
Tandems are expensive; a good tandem can cost more than twice as much as a good single of the same quality. A tandem is for two people, and both of you should be involved in the selection. When you are ready to buy a tandem take the time to find a shop that has experience selling tandems. A good bike shop may not be a good tandem shop. Look for a place with several tandems on display, and one that carries several different brands. See if you can get an extended test ride. It is harder to fit two riders to a tandem than it is to fit a single rider to a single bike. Do not settle for anything less than a perfect fit for the both of you. If you do you may end up riding that tandem alone. Having enough room for the stoker is especially important. Consider getting a suspended stoker seatpost, even if the captain does not have one, because the stoker’s ride is harsher.
You may want to get a used tandem as your first tandem, you can save some money and get the experience you need to make an informed choice, when you get your perfect tandem. Some good websites for used tandems are the Tandem Club of America: http://www.tandemclub.org/classifieds.html, and Tandem Magazine: http://www.tandemmag.com/classified/. So go shopping and find a tandem both of you are comfortable with, then check the websites for a good used version of the same bike.
Bikes often appeal to the gearhead in us, tandem also do, but even more so; there is just so much more to tinker with in a tandem. So have fun.