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• Keep your URLs to fewer than 115 characters.
• Research shows that people click on short URLs in search results twice as often as long ones. Shorter URLs are also easier to share on social sites like Twitter and StumbleUpon.
• Long URLs can look like spam. As the URL gets longer, the ranking weight given to each word in the URL gets spread thin, and becomes less valuable for any specific word.
You can manually check the character count of all your URLs to make sure they’re not too long. TheAboutUs Site Report can do it automatically, and point out any URLs that are longer than 115 characters.
• Don’t use more than a few query parameters in your URLs.
In a URL, a ? or & indicates that a parameter (like id=1234) will follow.
Here’s an example of an okay URL (the kind you use to track your marketing in Google Analytics with 1 query parameter: http://www.example.com/page?source=facebook
Bad URL with too many query parameters: http://www.example.com/product?id=1234567&foo=abc123def&color=yellow&sort=price
Too many query parameters can cause search engine robots to enter a loop and keep crawling the same pages over and over again. You could end up with search engines failing to index some of your most important pages.
• Use hyphens instead of underscores in your URLs.
Search engines see underscores as a character. This means that your keywords will be seen as a single long keyword, and you’ll lose any SEO benefit they could have incurred. A hyphen, however, is seen as a space that separates words. Hyphens are better for SEO because they allow search engines to interpret your web page as relevant for more keyword phrases. That said, Wikipedia’s links have underscores, and they seem to be doing okay in search results 
Also, people can’t see underscores in a URL when the link is underlined, as many links on the Web are. So hyphens are friendlier for people, and make your site more usable.
So… example.com/adorable-kitten-pics is better than example.com/adorable_kitten_pics
• Keep all of your important content less than 3 subfolders deep.
A subfolder is a folder that is visible in a URL between two slashes. For example, in http://www.example.com/articles/name-of-page, articles is a subfolder and name-of-page is an article in that subfolder.
When it comes to subfolders, search engines assume that content living many folders away from the root domain (like example.com) is less important. So it’s best to organize all of your important content so each URL has no more than two subfolders.
Here’s another way to think about it: Make sure your URLs have 3 or fewer slashes (/) after the domain name. Here is an example URL that is a web page that is two subfolders deep: http://www.example.com/articles/foo/page-name.htm
Bonus: Using subfolders allows you to use “content drilldown” in Google Analytics to easily view data for all the pages in a given subfolder.
• Don’t have too many subdomains.
A subdomain, or directory, is something that comes before the domain name in a URL. For example: http://blog.example.com. Technically speaking, www. is actually a subdomain.
Too many subdomains can cause problems for search engine optimization. For more information, read Multiple Subdomains: Classic SEO Mistake.
