Turbaned gurus, sing-song mantras and bodily contortions . . . the promise of true enlightenment and omphaloskepsis (contemplation of the naval) completes the cliche. But don't knock yoga till you've tried it, and then only with respect.
Yoga means to bind together -- variously joining sun and moon, left and right, male and female, and any number of yins and yangs -- through ascetic techniques of meditation and exercise. The goal is physical and mental balance.
Indian Hatha' yoga is best known to Westerners. Double-jointedness isn't a prerequisite, but the classic lotus position, cross-legged on the floor, soles-up on the inner thigh, either comes naturally or doesn't.
Then there are more magical/mystical varieties of yoga for which people quit jobs and polite society and retreat to the Himalayas. But not everyone follows a spiritual guide beyond the Beltway; they'd rather take up the discipline at a local ashram or the Y.
Committed practitioners claim yoga leads to intuitive awareness, spiritual harmony, perfect concentration. Others use it to lose weight or quit smoking. Some just like the lift they get from yoga asanas (positions) better than breaking into a sweat with pushups. In any case, it can't hurt, if done in moderation and with proper guidance.