webserver and application server

http://www.diffen.com/difference/Application_Server_vs_Web_Server

http://www.differencebetween.com/difference-between-web-server-and-vs-database-server/

http://www.acunetix.com/websitesecurity/webserver-security/

http://www.questpond.com/

http://blog.csdn.net/keyeagle/article/details/6723408