The first excerpt and link is to a great article of Liz Greene, which makes a good overview of the subject to my mind. Although the article is about composite chart, it explains it in relation to synastry chart and birth chart. I also thought it might give something to those, too, who are not so familiar with astrology as might the other links as well.
The Composite Chart by Liz Greene at astro.com
“One plus one equals three
The concept behind the composite chart – or, as Erin Sullivan sometimes calls it, the compost chart – is that it represents the relationship itself as a third factor. Two people create a third thing between them. The composite chart is like an energy field, which affects both people and draws certain things out of each individual as well as imposing its own dynamics on both.
The composite doesn’t seem to describe what either person feels about the other. In this way it is very different from synastry, which describes the chemistry between two people in terms of how they affect each other. When we are exploring the synastry in a relationship, we say, “Your Venus is on my Mars. You are activating my Mars and bringing a Mars response out of me, and I am activating your Venus and invoking a Venus response in you. Consequently we feel a certain way about each other.” When we are looking at a composite chart, we are not exploring what two people activate in each other or feel about each other. We are interpreting the energy field they generate between them. The composite chart is like a child, a third entity which carries the genetic imprints of both parents but combines these imprints in an entirely new way and exists independently of either of them.” Read more here: http://www.astro.com/astrology/in_composit_e.htm
Synastry & Relationship Astrology
Image: Marc Chagall – Over the Town /wikiart.org