What is a sun salutation?

Surya Namaskar, Salute to the Sun or Sun Salutation, is a flow sequence of asanas (postures) linked together by the breath. There are a few variations on sun salutations, see this short clip of a Sun Salutation A flow.

Why 108?

The number 108 is considered sacred in many traditions and also surfaces throughout yoga, nature, and astrology.  For yogis 108 is considered a code for awakening and is said to represents the unity and wholeness of existence. In yogic traditions to celebrate the passing of Spring or Autumn equinox (often New Year and the solstices too) 108 sun salutations are completed. By practicing chanting, breath work, or asana in rounds of this sacred number, the ancient yogis believed we could align ourselves with the rhythm of the creation/nature. Also present in Buddhism, we see temples have 108 steps representing the 108 steps to enlightenment, and in Japan, at Zen Buddhist temples, a bell is chimed 108 times at the end of the year closing a cycle. 108 seems to have an essence that connects us to the whole and serves as a reminder of the wonder and interconnectedness of the universe.