在网上看到几篇比较好的文章很详细讲解了Form、Membership、以及Identity身份认证

Form身份认证: http://www.cnblogs.com/fish-li/archive/2012/04/15/2450571.html
Membership: http://www.cnblogs.com/jesse2013/p/membership.html
从Membership到Identity: http://www.cnblogs.com/jesse2013/p/3583308.html
Identity 登录原理 http://www.360doc.com/content/14/0802/22/14692560_399009178.shtml