Search Engines explained
There are the following three parts of Search Engine
a) Crawler
b) An index
c) A search interface
Now here are the three explained in details
Crawler (A.K.A - Spider)
Crawler goes through the web following the links.It updates the web pages and adds new when it comes accross them . The web pages that are updated every now and then are crawled rapidly.The frequency of getting crawled varies proportionally to the no. of times the site is updated.
The Index
The Index is the place where the spider-collected data are stored.The user who searches for some particular keywords gets the data stored here not on the web. 
Search Interface
The Search interface is used to find the most relevant data to the one which is being searched .The search algorithm also comes to the fore here.Both combined,fetch the most desired results to the user.The process that they use to get it to their root words and all is very complex and it is better not to know about them at this phase.We will learn later.
Good Time,
Swastik
