Servlets are similar to Java applets that are run on the server side. They are used for running interactive web applications. They are considered to be platform independent and to have improved performance over CGI programs.
JavaServer Pages (JSP) is an extension of servlets; they are used to generate webpage content using XML-like tags and scriplets written in Java.
The official Tomcat website is at http://jakarta.apache.org/tomcat/.
Information about the the Servlet 2.3 implementation can be found via http://java.sun.com/products/servlet/. And specifications for JavaServer Pages 1.2 are available via http://java.sun.com/products/jsp/.