What is the difference between a webserver and an application server? What funcionality do typical application servers provide?

A web server handles the interaction with the client’s web browser. Application servers are used to maintain a pool of processes for handling requests. Typically, they are the middleware tier between the web server and the data sources such as database systems. Application servers eliminate the problems with process- creation overload and can also provide extra functionality like abstracting away heterogeneous data sources and maintaining session state information.

When is an XML document well-formed? When is an XML document valid? 

An XML document is valid if it has an associated DTD and the document follows the rules of the DTD. An XML document is well-formed if it follows three guide- lines: 

1.  it starts with an XML declaration, 
2. it contains a root element that contains all other elements and 
3. all elements are properly nested.