You can sync to the cloud and access files from the cloud OR you can actually choose files/folders to sync across hardware. For example, if I say that I want to sync my music folder across my Macbook Air and my Thinkpad, any time I make an update on one machine, it flows through to the other machine's hard drive. Similar with documents. If I choose to sync a Word document across machines and I edit it and save on one machine, in seconds it updates on the other machine should I open it there. It's all seamless - happens in the background without me having to push it from machine to machine.
Re: the API point, SugarSync has had one since 2010: http://www.sugarsync.com/developer. Haven't developed with it at all nor have I used an app that uses it, so can't speak for its flexibility.
P.s. I swear I'm not a SugarSync employee - just a very satisfied user since 2008 ;-)