Shadowsocks ( http://shadowsocks.org ) is an open source socks5 proxy which ,according to their official site, is designed to protect your internet traffic. in English, it will encrypt the traffic between you and the servers, so the ISP can not spy on you. once the ISP do not know what are you doing, they will not block the traffic, so you can bypass the restrictions by the ISP.
The main difference compare to VPN is Shadowsocks is not global, which means not all your traffic will go through the servers. VPN is global, once you connect to VPN servers, all your traffic will go through the servers, the main benefits of Shadowsocks is you can choose which traffic will go through the server and which will not. for example , i am in China, i want to watch China-only movies on Youku.com and meanwhile i want to use twitter, if i use VPN, i have to connect to a VPN server that is not in China, and once i connected, i can not watch videos on Youku.com or the speed is horrible . if i use Shadowsocks, the problem resolved, because the Youku.com traffic will use the local network which is not go through the servers and the twitter traffic will go through the server.
Shadowsocks is becoming more and more popular in China these days, however most of the Chinese build the proxy server themselves , there are a few China based teams provide Shadowsocks service, i didn’t see many service provider outside China, the “openweb” model of Astrill looks like socks5 proxy but i don’t know which technology they use.