Answer for question no:1

difference between web container and web server

Web container is nothing but a folder where a web site's files and folders are stored. When you accessing a web site all files are stored in seperate folders.

Web server is known as a server which processes the user requests and sends reply to the users.

Answer for question no:2

What is Tomcat ? is Tomcat web container or web server ?

Tomcat is a web server. All of the J2EE web based projects will be stored in the location Tomcat> Web-apps><our folder> .
Here our folder is where we maintaining our project files. To run this project we must type in our browser ashttp://localhost:8080/<our folder>/<first accessible file name>.
Here 8080 is the port number. This is the default port number. You must check the port number while installing tomcat.

Answer for question no:3

Out of Interface and Abstract class Which one of we should use and why ?

I do not know the exact answer. But my suggession is interface must have variables with modifiers static and final. I think this only may be the reason.