Service is like activity , but no UI.
related links:
http://www.cnblogs.com/zhangdongzi/archive/2012/01/08/2316711.html //good
http://blog.csdn.net/ryantang03/article/details/7770939
android+Service
原文：http://www.cnblogs.com/zhongyuan/p/4217516.html
