Summary
BDR is a rising star in the world of PostgreSQL replication. Currently, it is still under heavy development, and we can expect a lot more in the very near future (maybe even when you are holding this book in your hands).
BDR is asynchronous and multi-master, and allows people to run geographically distributed databases. It is important to note that BDR is especially useful when the odds of a replication conflict are low.