写得太好，还是忍不住把链接mark一下：
http://blog.echen.me/2012/01/03/introduction-to-conditional-random-fields/
http://lhdgriver.gotoip1.com/%E6%9D%A1%E4%BB%B6%E9%9A%8F%E6%9C%BA%E5%9C%BA%E7%AE%80%E4%BB%8Bintroduction-to-conditional-random-fields/