Lsyncd(1) watches local directory trees through an event monitor interface (inotify, fsevents). It aggregates and combines events for a few seconds and then spawns one or more processes to synchronize the changes. By default this is rsync(1). Lsyncd is thus a light-weight asynchronous live mirror solution that is comparatively easy to install not requiring new filesystems or blockdevices and does not hamper local filesystem performance.
Rsync+ssh is an advanced action configuration that uses a ssh(1) to act file and directory moves directly on the target instead of retransmitting the move destination over the wire.
Fine-grained customization can be achieved through the CONFIG-FILE. Custom action configs can even be written from scratch in cascading layers ranging from shell scripts to code written in the lua(1) language. This way simplicity can be balanced with powerfulness. See the online manual for details on the CONFIG-FILE http://code.google.com/p/lsyncd/wiki/Lsyncd20Manual
Note that under normal configuration Lsyncd will delete pre-existing files in the target directories that are not present in the respective source directory.