In 1850, ships from Australia became the first to carry cockatiels to Europe, where they've been some of the most popular pet birds ever since.
Originally, they were all gray, excepting the yellow heads and faces of cockatiel males, but in the 150 years since their initial importation, selective breeding has produced cockatiels in almost every color. Their most noticeable characteristic is the crest, which crowns their head and rises and falls depending on the bird's mood. They also have unusually soft feathers -- called powder-down feathers -- under their wings and possess long, pointed tail feathers.
If you decide to adopt a cockatiel, the ideal age is 6 to 8 weeks old. This is the time during which you'll best be able to bond with your bird. There's no way to determine the sex of a young cockatiel until they're around 6 months old, after molting their juvenile plumage to make way for adult feathers. When the new feathers grow in, females have yellow spots on the underside of their wing feathers, while males have no distinguishing marks.
You'll need to house your bird in a cage that's at least 18 inches long, with horizontal bars to aid climbing, a grill on the bottom, and a concrete perch to help keep its nails trim. Cockatiels thrive on a simple diet of high-quality seed mix (low in sunflower seeds) that's mixed with a good commercial pellet food, as well as fresh greens, vegetables, and fruits. Their water should be supplemented with vitamins, and you should affix a cuttlebone on the side of the cage to increase the calcium in their diet (females who lay lots of eggs will benefit from an additional liquid-calcium supplement).
It's extremely important that the cockatiel's water dish be cleaned out with soap, water, and a scrub brush each day. It should be cleaned until you'd feel safe drinking from it -- a rule of thumb that holds true for any variety of bird.
Finally, cockatiels are very vain and love to groom themselves, which you can facilitate by moistening their feathers daily with warm water from a mister. To promote their emotional health, give them a different toy to play with each day.