Except in religious or spiritual literature, this word is almost always used incorrectly to mean something like mystification, fuzzy-wuzzy, or airy-fairy. But contrary to popular misconception,
mysticism means nearly the opposite of those common and ignorant connotations. In reality, mysticism is the most empirical aspect of religion because it involves direct experience of the Divine, rather than faith in the Divine. The direct mystical experience of God is the experience of yoga or oneness with the Divine.
Mysticism is becoming increasingly important in the world because of the decline of traditional religions that rely on faith and the rise of spirituality to take their place. In the meantime, mysticism is a common denominator of all of the great world religions and is an element that can bring religions closer together rather than separate them as do so many other aspects of religion.
An excellent little book about mysticism is Walter Stace's
The Teachings of the Mystics (New York: New American Library, Mentor, 1960); if you can't find it in a bookstore, try a library or http://www.bookfinder.com.