Bernard was a Cistercian monk and mystic. He was a very attractive figure, so charismatic that it was said that mothers would cover the ears of their children lest he tempt them to join the monastery. This popularity has only grown over the centuries since his death, and may be part of the reason for the popularity of the Cistercian order. The Cistercians, like other orders of the time, advocated a return to a more devout and simple life, but reaffirmed the rule of St. Benedict in doing so. They are now divided into two observances, the common and the More Strict (sometimes called Trappists). Thomas Merton, the 20th century mystic, was a Trappist. The Trappists had a vow of strict silence, and abstinence from flesh, fish, and eggs. Anyway, Bernard is the early influential figure in this order, although he did not start it.
For Bernard, theology is divided (at least practically) into dogmatic and mystical. As can be seen from the chart, his dogmatic theology emphasized freedom and renewal. His mystical theology emphasized love.
He draws on the traditional mystical path of purgation, illumination, and union, although he expands this to seven and even twelve steps in some places. The image of these stages is the kiss, or rather the successive kisses of Christ. Note the erotic connotations. He goes as far as saying that even carnal love is part of the mystical path. It is not something to be avoided or downplayed, but must be redeemed.
The larger emphasis on love comes from Bernard's conviction that the problems of the self and the world are not problems of knowledge, but of will, and that love alone engages a person at a sufficiently deep level to bring about true conversion <from "Light from Light">. The way to overcome the self-love that prohibits us from true love is not simply to try to love everything, but rather to love God properly. The best way to do this is not to just reflect on God's love in general, but on Christ's love in particular. If you truly internalize Christ's love, you will be able to properly love yourself and the world.
Bernard's writings, therefore, are lyrical in their expressions of love for Christ. The end result of all this is that Bernard's mysticism does not emphasize how we know God, but how we love God. He still is concerned about understanding, but "affective mysticism" has its beginning here.