SEO friendly URLs are purely structural URLs that do not contain a query string and instead contain only the path of the resource (after the scheme [e.g., http] and the authority [e.g., example.org]). This is often done for aesthetic, usability, or search engine optimization (SEO) purposes. Other reasons for designing a friendly URL structure for a website or web service include ensuring that individual resources remain under the same URL for years, which makes the World Wide Web a more stable and useful system, and to make them memorable, logical, easy to type, human-centric, and long-lived. friendly URLs, being a subset of URLs, are also a subset of URIs

Examples of "friendly" versus "unfriendly" URLs.

Unclean URL	Clean URL
`http://example.com/index.php?page=foo`	`http://example.com/foo`
`http://example.com/index.php?page=consulting/marketing`	`http://example.com/consulting/marketing`
`http://example.com/products?category=2&pid=25`	`http://example.com/products/2/25`
`http://example.com/cgi-bin/feed.cgi?feed=news&frm=rss`	`http://example.com/news.rss`
`http://example.com/services/index.jsp?category=legal&id=patents`	`http://example.com/services/legal/patents`
`http://example.com/kb/index.php?cat=8&id=41`	`http://example.com/kb/8/41`
`http://example.com/index.php?mod=profiles&id=193`	`http://example.com/profiles/193`

SEO-friendly URLs can:

Describe your content
Help you avoid duplicate content
Avoid having your pages trigger spam filters
Make your pages easier to share, remember, and type into a browser
Help your Web visitors navigate your site
Increase search engine ranking for sought after keywords

The way that a search engine robot reads your URL can be different then how a human reads it. While you are structuring your website it is very important to create a URL structure that will please your mechanical visitors as well as your human ones. URLs that are too long, have underscores, capital letters, arbitrary numbers or even just look a little weird can cause problems in a couple of different ways.
Bad URLs can:

Hinder usability
Create duplicate content
Expose file extensions and other security vulnerabilities

URL Capitalization
Capitalization directly effects the way that search engines index your site. While domains are not case sensitive by themselves, URLs are. So when a search engine sees www.yoursite.com/Section-One/ and www.yoursite.com/section-one/ it indexes them as two different web pages. Two different versions of the same page with the same content will be caught in Google's duplicate content filter. This can degrade your website's SEO value.

Similarly if your URL has capital letters, and a user links to or shares the lowercase version, it could end up in a 404 error, indicating that the page was not found.

The shorter and cleaner your URL is, the better as far as search engines are concerned. Longer query strings (associated with dynamic URLs) are confusing to search engine spiders. Keeping it shorter and cleaner (i.e. http://www.widget.com/4/basic.html vs. http://www.widget.com/cgi-bin/gen.pl?id=4&view=basic )

A URL like http://www.example.com/4/basic.html can be indexed much easier, whereas its dynamic form, http://www.example.com/cgi-bin/gen.pl?id=4&view=basic, can actually confuse the search engines and cause them to miss possibly important information contained in the URL, and thus preventing you from getting the expected ranking.

a URL that contains "seo_techniques" will be shown by the search engine only if the user searches for seo_techniques (but this kind of search is rarely performed); whereas searches for "seo", "techniques", or "seo techniques" give your "seo-techniques" containing URL a better chance of being displayed on the SERPs.

File Extensions in URLs
Having file extensions, like .html, .php, .asp and so on, at the end of your URLs does not appear to have a direct effect on your web page's search engine ranking.
URLs that end in .html are favored by search engines. There are exceptions to this so make sure that you know your system well and that it’s friendly with search engine requirements.

File Structure
The architecture of your site is very important for a lot of reasons. The way your files are structured makes a difference in how your website is crawled by search engines, as well as surfed by a user. A good rule of thumb to follow is to try to have your most important content within two clicks of the home page. Pages that are placed closer to your home page will gain more rank overall.

Proper folder structure should be used, so any pages nested under /products/ should include the folder in their URL, e.g. www.yoursite.com/a-great-product/ should be at www.yoursite.com/products/a-great-product/. This aids usability in that users always know where they are located in your site.
URL best practices are one of the first things someone will discover when they begin to explore SEO. It is a very simple idea although some webmasters and CMS systems still get it terribly wrong from time to time. The good news is that once your URL issues have been corrected it is possible to create 301 redirects from the old URL versions to the new ones.

Spammy URLs
If you still have certain doubts about your URL, go to a resource like seomoz.com There are spam detection tools that will help weed out things like spam words, and give you tips on hyphens, sub domain depth, domain length and digits. Again, all of this information is freely available online.

Hyphen vs. Underscores
The websites that contain an underscore in their URL are starting to become scarcer. The general belief now is that a user who is doing a Google search for “blue widget” will get better results from http://www.blue.com/blue-widget than http://www.blue.com/blue_widget. Times change, and these protocols change as well.

How to Rewrite an URL
The principle of URL rewriting is actually setting a "system" on the host server that will allow it (i.e. the server) to know how to interpret the new URL format. What actually happens when one decides to rewrite the URLs of a certain Web site is masking the dynamic URLs with static ones. This means that the URLs that previously contained query strings with elements such as "?", "+", "&", "$", "=", or "%" will contain the more search engine friendly "/" (slash) element and present themselves in a simplified form. To help you with cleaning your URLs, there are rewriting tools and engines, some free of charge, other fee based. Online / Open Source Tools

This is the most common non-fee-based rewriting engine. It is a module from the Apache HTTP Server that allows the easy manipulation of URLs. The use of this module requires enabling the RewriteEngine on your Apache server. Then, rewrite rules must be defined, and you can even set conditions for each rule, thus allowing you to rewrite requests as they come in.
In terms of SEO, mod_rewrite can be helpful if you have complex URLs that contain more than 2 parameters. In other words, if one of your dynamic URLs is accessed, the mechanism behind mod_rewrite will "translate" it into a shorter, friendlier, static-looking URL.

Fee-Based Tools

ISAPI_Rewrite The Internet Server Application Program Interface (ISAPI) is another URL manipulation engine that functions in a similar way to Apache's mod_rewrite, with the difference that it is designed specifically for Microsoft's IIS (Internet Information Server).
IISRewrite IISRewrite is a stripped down implementation of Apache's mod_rewrite modules for IIS. It is a rule-based rewriting engine that allows a Webmaster to manipulate URLs on the fly in IIS.

URL Examples
Below are some examples of how URLs can look before and after rewriting:
Example 1:

Dynamic URL:

(before rewriting)

Static URL:

(after rewriting)

Example 2
:

Dynamic URL:

(before writing)

Static URL:

(after writing)

Conclusions
URL rewriting can bring you on the right track in the race for SEO (combined with other techniques that you may use for this purpose). But be aware that rewritten (and, presumably, better looking and more effective in terms of search engine ranking) URLs cannot substitute a poorly designed Web site.
So don't expect miracles. Nevertheless, when you decide that your site needs a makeover and start rewriting your URLs, make sure that:

Keep them short, keep them clean. (to increase usability),
You use lowercase letters rather than uppercase ones (to avoid case sensitive situations),
Keep the underscores out and replace them with dashes..
Keep as many parameters as possible out of dynamic URL’s. Not only is it better from a search engine standpoint, but it makes it more difficult for the site to be hacked.
The technology you have used cannot be detected in any of your URLs (to prevent possible hacker attacks).

SEO-friendly URLs For Wordpress Blog

Step 1: Login to your WordPress account. It should take you to the Dashboard area.
Step 2: Click Permalinks under Settings tab.
Step 3: Tick the URL structure you want to use.
We recommend date based permalinks or post ID and postname. You can also go choose Custom Structure because they make sense to users and search engines. Other seasoned WordPress developers recommend /%category%/%postname%/ although Help Pages in WordPress discourage that since WordPress might confuse it with Pages URLs who might share the same initial texts as the category. This little confusion might cause delay in loading your page while WordPress is trying to distinguish if it will take you to a page or if the link is specifically referring to a blogpost.
SEO Friendly Permalinks WordPress

You can read Using Permalinks to find out more how to use go about using a custom URL structure if you prefer that option. In any case, the ones I stated above will work just OK.

Other Video tuteriol For Wordpress Blog

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