How enter prise search works in sharepoint 2010

Question

In my organization enterprise search is configured ,
-- sharepoint farm has
-- two index servers ( share point server search  service is  configured in this servers)
two web front end servers(sharepoint foundation web applications, and  share point server search  service is  configured in this servers)
- two  application servers( central admin service is running)
- one sql server
here i am new to this area
here how entr prise search works,  why index servers used
index data stored sepratly in thi servers?
and when user query for any documentid where it will search initially
i know the concept of search there are 3 things
1) query
2)index
3)crawler
adil

Answer 1

Hi,
In Microsoft SharePoint Server 2010 Search, index partitions are groups of query components, each of which holds a subset of the full text index and which return search results to the query originator. Each index partition is associated with a specific property
database containing metadata associated with a specific set of crawled content．
The index server manages the crawling and indexing of SharePoint Server content and maintains the index file. The index service works with the query service to let you search SharePoint Server content.
Every SharePoint Server deployment must have at least one index server. Additional index servers may be necessary as the deployment grows. There can only be one index server per server farm.
If you use the web front end as your query server.  This tells the index server to propagate its index to the WFEs that are set as query servers so that they have a local copy of the index.
When someone does a search (this is done on the WFE), then that WFE will search itself locally instead of going across the network to query the index server.  
This increases speed at the time of query, but it of course introduces additional overhead in terms of having multiple full copies of the index on the network and the network demand of propagating those index copies all the time. 
If you select the query role on your index server, then the index will not get propagated and all searches will query the index server across the network.  
For more information, you can refer to the following article.
http://www.sharepointpitstop.com/2012/08/sharepointsearch-index-query-crawl.html
Thanks & Regards,
Jason
Jason Guo
TechNet Community Support