Is tomcat webserver or application server

tomcat is bot a application & webserver

how come tomcat is a application server;we can't deploy ear in tomcat naa..

You can deploy war files to tomcat
http://www.jguru.com/faq/view.jsp?EID=123229
http://jakarta.apache.org/tomcat/tomcat-4.1-doc/appdev/deployment.html
http://www.onjava.com/pub/a/onjava/2001/04/19/tomcat.html

i am talking abt ear not war...(jar+war=ear)i can't work ejb's in tomcat naa;

Check openEJB for that
http://www.onjava.com/pub/a/onjava/2003/02/12/ejb_tomcat.html

i know openejb;but tomcat can't directly support ejb's;we have to install third party jar's

Yes thats true

my assumption is that whatever server supports ejb's that's we call application server

but i see in some of the articles or in forums why tomcat is referering as application server

J2EE Application Server should have two containers. a servlet container and another an ejb container

Tomcat is not a full  j2EE application server by itself (it can serve static content as well as dynamic content.)
Tomcat is servlet container that is subset of the J2EE Application Server.
JBoss is an ejb container which with Tomcat can work as J2EE Application Server

>> but i see in some of the articles or in forums why tomcat is referering as application server

No. it's not. People can get quite confused about this (including article writers!).

>> J2EE Application Server should have two containers. a servlet container and another an ejb container
Many J2EE containers actually consist of 3 elements, a web server for static content,  a servlet container for JSP/servlets and the the rest of the application server which manages with the EJB and ear side of things.