What is this?
This is a simple way to synchronize a folder hierarchy full of photos, some of which are marked for publication, between a local disk and a photo sharing service.
What services are supported?
Picasa Web Albums.
The first service to be supported was Zenfolio, but I do not have a Zenfolio account any longer (see this post for historical background).
One of the goals of the design was to make adding more services easy: service interfaces are general enough and at the same time allow quirks of specific services to be accommodated.