What is a web application?

A web application is a dynamic view of an application in a web server or application server.
They are mainly two types
Presentation-oriented:

A presentation-oriented web application generates interactive web pages containing various types of markup language (HTML, XHTML, XML, and so on) and dynamic content in response to requests. This can be achieved by technologies such as JSF, Servlet, JSP, Spring etc.

Service-oriented:

A service-oriented web application implements the endpoint of a web service. These are often consumed by the above said application. This can be achieved mainly with the help of Webservice technologies. 

A web application consists of web components; static resource files, such as images and cascading style sheets (CSS), Code that generates dynamic content such as servlet and Java classes etc. 

The process for a java web application creation can be summarized as follows:

1.    Develop the web component code.
2.    Develop the web application deployment descriptor, if necessary.
3.    Compile the web application components and helper classes referenced by the components.
4.    Optionally, package the application into a deployable unit such as war
5.    Deploy the application into a web container.
6.    Access a URL that references the web application.

What is a servlet?

Servlets  are platform-independent Java classes which implements the Servlet interface or extends any abstract class which has this Servlet interface implementation defined in javax.servlet package. This has three essential methods for the life cycle of a servlet – init(), service(), and destroy(). They are implemented by every servlet (defined in SDK or self-defined) and are invoked at specific times by the server. Servlets interact with Web clients via a request/response paradigm implemented by the servlet container.

The hierarchy is as follows.

                       public interface Servlet

public abstract class GenericServlet  extends java.lang.Object

   implements Servlet, ServletConfig, java.io.Serializable

public abstract class HttpServlet extends GenericServlet
              implements java.io.Serializable

What is a Servlet Container?

Servlet container or Web container is a standalone component or the component of a web server that interacts with Java servlets. The container implements Java Servlet Specification whereas the servlet is a java class which conforms to Java Servlet Specification. Thus A web container is responsible for managing the lifecycle of servlets, mapping a URL to a particular servlet based on configuration and ensuring that the URL requester has the correct access rights.

All servlet containers must support HTTP as a protocol for requests and responses, but additional request/response-based protocols such as HTTPS (HTTP over SSL) may be supported.

In Servlet 2.4 Specification the sequence of events which will occur is given as,

1. A client (e.g., a Web browser) accesses a Web server and makes an HTTP re-
quest.
2. The request is received by the Web server and handed off to the servlet con-
tainer. The servlet container can be running in the same process as the host
Web server, in a different process on the same host, or on a different host from
the Web server for which it processes requests.
3. The servlet container determines which servlet to invoke based on the config-
uration of its servlets, and calls it with objects representing the request and re-
sponse.
4. The servlet uses the request object to find out who the remote user is, what
HTTP
POST
parameters may have been sent as part of this request, and other
relevant data. The servlet performs whatever logic it was programmed with,
and generates data to send back to the client. It sends this data back to the client
via the response object.
5. Once the servlet has finished processing the request, the servlet container en-
sures that the response is properly flushed, and returns control back to the host
Web server.