Yoga is a form of exercise and involves stretching only
Yoga is a complex integrated system, which has a history of 5,000 or more years. It has eight parts to it, with the physical aspect being just one. These eight parts include yoga do's (Niyama), yoga don'ts (Yama), physical exercises (asana), breathing exercises (Pranayama), sense control (Pratyahara), concentration (Dharana), meditation (Dhyana) and divine union (Samadhi). Although the physical aspect of yoga, which is called asana, is an important part of yoga, yoga as a whole is certainly more than just a series of postures and movements.
The yoga postures may look like the stretching exercises found in some sports. The postures are not an end in themselves, but are meant to help prepare one's body for mental and spiritual development.