What is shell ?

Shell is a tool using that attacker can deface the web application. There are mainly two types of shell PHP shells and ASP shells. PHP shells works only on php web application and ASP shells woks on asp web application. After uploading the shell if you want to deface the web application then simply find the index.html or default.asp and change the coding with your deface page. If the server is Linux based then you can root the server using exploits and you can defaced all the web application hosted on the same server. But if the server is Microsoft then you cant root the server because there is no root.
Using the shell you can do email spoofing means you can send email from any email address to any one.
If you get the admin access in to the site and you have uploaded your shell then dnt forget to use the email spoofing tool.