They're called recumbents, which if you look the word up in the dictionary, is defined as "lying down; reclining," a description that nicely sums up the riding position on a recumbent. And, is the reason, when you're pedaling one of these rolling chaise lounges, that people routinely smile, wave, and holler, "cool bike!"
Though the design seems to buck tradition, recumbent bikes actually have a long and colorful history. They first appeared at the turn of the century. Then, in the thirties in France, they created a stir by winning some major races, which quickly got them banned because the race officials (spoil sports) said they offered an unfair advantage due to their wind-cheating profile. This decision kept the bikes underground for years. But, you can't keep a good thing down, and today, recumbents are more popular than ever.
Obviously, the major difference between recumbents and conventional bikes is the seating position. You're not sitting so much on top of the seat, as you do on a regular bicycle, you're sitting in the seat, which features full support for your back along with your bottom. The seats are also widely adjustable (often, even the back angle can be changed) for tuning fit and comfort. So, goodbye numb bum!
Because you recline to ride, there's no pressure on your hands and arms and no stress to your back, neck and shoulders. Vision is improved because your head is naturally held upright where you can easily look out and up. Most cyclists who try recumbents come back from rides having noticed things on their regular routes that they had always missed seeing before.
There are many different types of recumbents and the bikes vary in design far more than conventional models. This makes it trickier to find the right one to buy. Interestingly, once you get into recumbents you may end up buying several because they all ride different enough that you can enjoy owning more than one as many recumbent aficionados do.
As far as disadvantages to recumbents, there aren't many. Generally speaking, they tend to be slightly heavier than standard upright bikes, which translates to slightly slower climbing speeds on some models. But, on a 'bent, you go faster on the flats and downhills, so it evens out. Some cyclists worry that the low profile makes these lowriders invisible in traffic. But, this is debated. The bikes' unusual appearances is actually more attention-getting. Plus, if you're concerned about safety, it's easy enough to mount a flag on your machine to increase visibility.
The best way to check these bikes out is to test ride one. You'll feel strange at first because you're used to the balance point of an upright. But, after a few wobbly feet, you'll gain confidence and begin to cruise and see how much fun these machines can be.