Correct body alignment allows the body to develop harmoniously and in an anatomically correct way so the student suffers no injury or pain. As all bodies are different and people have different weaknesses and strengths props are used to help the body into the correct positions required.
Today Iyengar is one of the most widely practiced yoga techniques
in the West. It was developed in India by B.K.S. Iyengar who systematised
over 200 asanas (positions) and 14 different types of pranayamas (breathing
exercises) ranging from the simple to the incredibly difficult. These have
been structured and categorised so as to allow a beginner to progress from
basic postures to the most advanced as they gain flexibility, strength and
sensitivity.
Other interesting links:

MAGAZINE
About Yoga
YOGA
- genearal
Yoga with Yoga Travel
What is yoga
The difference between a holiday & a retreat
Why go on a yoga holiday?
Which style to choose
Different styles and different teachers
What can Yoga do for me?
The Yoga of Flying
Yoga for preventing holiday anxiety
What is meditation
What is pranayama
Yoga and Diving
A history of yoga
Yoga glossary
Yoga & Spirituality
The Guru System
Yoga and Tantra