Difference between web-server and servlet container

What is the difference between "A Web Server" and "A Servlet Container"?
In general are all Web Servers Servlet(Web) Containers too or vice versa?

I have been using Apache Tomcat for JSP and Apache for PHP but unable to figure out the exact difference between those two, where exactly lies the difference?

I tried searching previous threads but could not find much.

Thanks! :-)