It allows you to share & synchronize in almost real time a folder content between multiple devices (PC, tablet, smartphone, ...) wherever they are as long as they are accessible thru a network.
BTSync is quite simple to setup and allows you to create a secured & distributed private network with whoever and whatever device you want. All synchronization job accross devices is done automatically in the background, using a peer-to-peer (P2P) protocol. The connexion between devices is done thru UDP, it handles NAT traversal and UPnP port mapping.
This article explains some BTSync usages (smartphone on-the-air access, server backup, ...) and some BTSync configurations that will allow you to use BTSync anytime, from any device (PC, smartphone, tablet, ...).
You'll then create you self-hosted Dropbox or Google Drive replacement. Some huge possibilities are opening ... your main limit will be your own imagination !