webserver vs application server

1) both refer basically to the same thing..

2) application server can be taken as the super set of an web server.

3) Example of web server are IIS, Apache, and example of application server are BEA Weblogic, IBM web sphere etc.

4) Both the web server as well as the application server can talk to the database server.

5) Only when the business logic, or transaction involved is on the higher side, an additinal layer in the form of application server is added in between the web server and the database server.
i) usual 3-tier architecture involves -
client (browser) --> web server (apache, IIS etc) --> database server (mysql, oracle etc)
ii) 4-tier architecture involves -
client (browser) --> web server (apache, IIS etc) --> Application server (Weblogic, websphere etc) --> database server (mysql, oracle etc)