Embedded web server isn't new term. Web server is jus a method of delivering web pages (or other contents) to the user.

Market is full of such systems allowing interconnection of domestic devices and other web-enabled appliances. It is hard to understand how microcontroller can drive web server. Embedded web server usually is highly optimized machine code. Usually implemented. In order to write optimized embedded web server it is necessary to write code from scratch.

What parts need to be implemented while creating embedded web server:
Microcontroller hardware;
Network protocols for embedded web server;
Implementation and optimisation techniques.
Microcontroller hardware is a computer on a chip which has program memory for code storage, EEPROM, RAM, I/O and other peripherals.
Network protocols can be used for embedded web server:
HTTP-document request/response
TCP-reliable communications
IP-low-level data transport
ICMP-diagnostics (ping)
SLIP-serial interface
Modem emulation

HTTP request is Hyper Transfer Protocol which defines a request – response mechanism for obtaining documents from web server. Web browse sends request to web server in multiline string format like:

GET /index.htm HTTP/1.0
User-Agent: Mozilla/4.5 [en]
(Win95; I)
Pragma: no-cache
Host: 10.1.1.11
Accept: image/gif, image/x- xbitmap, image/jpeg, */*
Accept-Encoding: gzip
Accept-Language: en
Accept-Charset: iso-8859-1,*,utf-8

In embedded systems all this information isn't needed. For instance we don't care about browser information, HTTP version isn't important because usually simplest HTTP interface is used. Simply you can trim the rest part of message and send line:
GET / HTTP/1.0
because most web servers interpret this as request of index.htm.