How Search Engine works?

It is really important to know how Search Engine works before doing any SEO work. because unless you know how it works? you are not able to do understand the real meaning of Optimizing the Search Engine.

so let's start our discussion from Search Engine work.Later we will discuss the Directory work structure.

                                

I would say we have a lot of Search Engines available world wide...google, inktomi, baidu(china), naver(korea), alltheweb. Does every search engine follows same strategy ?If so what is the pros  & cons.

Generally individually every Search Engine has their own algorithm. It follow their work based on them. Generally search engines update the database for 3 months.
So major goals and functions of Search Engine would be:
 1) Crawling of the web pages based on search engine algorithm.
 2) Indexing the web pages by selecting proper content on specific keyword.
 3) Responding to the user queries by relevant pages.
 4) Ranking of the pages .

Search Engine Vs Directories.

Lets us start our discussion with what is Search Engine? 
    
Search Engine  is internet based  tool that searches an index of documents for  a particular term, phrase or text specified by user.Other wise we say it is  a software program that searches a database  and reports information that is related to specific term.
Examples of Search Engines : Google, inktomi, altvista ,alltheweb and msn.
A Directory is website which looks just like a search engine but organised as a series of sections or categories  under which appropriate sites are filled.so Directory is divided into  specific categories and sub categories.
Examples of Directories : yahoo, dmoz.org, gimpsy.
So the main basic purpose of Search Engine & Directory is "to help people find websites of interest to them".

Let us discuss the what is difference between Search Engine and Directories:
1.Search Engines are run by robots where as Directories run by human Editors.
2.Search Engines are free to get listed in and they never charge for it.where as some directories are free and some will charge  money for each submission.
3. Search Engines are more popular and used because of their convenience.
All people have do is just type a keyword in search engine relevant sites come up.
                     vs
 Directories are less popular because of their way of organizing data. In Directories we have to go  through the specific categories and sub categories.
4.Search Engines some times disorganized when someone visits a website.but directories always have listings grouped together by topic.

What is SEO? Importance of SEO?

SEO( Search Engine Optimization). The name itself tells that optimizing the search of sites in search engines.

SEO is the act of modifying a website to increase its ranking in Organic (vs paid) , crawler - based listings of Search Engine or else we can say "SEO involves  altering  website code, content and presence in order to increase rankings in Search Engine. 

SEO plays a significant role in increase of SERP's for your site,blog or webpages of your company.