Simple-to-understand URLs will convey content information easily 
Creating descriptive categories and filenames for the documents on your website can not only help you keep your site better organized, but it could also lead to better crawling of your documents by search engines. Also, it can create easier, "friendlier" URLs for those that want to link to your content. Visitors may be intimidated by extremely long and cryptic URLs that contain few recognizable words.

URLs like (1) can be confusing and unfriendly. Users would have a hard time reciting the URL from memory or creating a link to it. Also, users may believe that a portion of the URL is unnecessary, especially if the URL shows many unrecognizable parameters. They might leave off a part, breaking the link.

(1) A URL to a page on our baseball card site that a user might have a hard time with.

Some users might link to your page using the URL of that page as the anchor text. If your URL contains relevant words, this provides users and search engines with more information about the page than an ID or oddly named parameter would (2)

(2) The highlighted words above could inform a user or search engine what the target page is about before following the link.

URLs are displayed in search results
Lastly, remember that the URL to a document is displayed as part of a search result in Google, below the document's title and snippet. Like the title and snippet, words in the URL on the search result appear in bold if they appear in the user's query (3). To the right i s another exampl e showi ng a URL on our domai n for a page containing an article about the rarest baseball cards. The words in the URL might appeal to a search user more than an ID number like "www. brandonsbaseballcards.com/article/102125/" would..

(3) A user performs the query [baseball cards]. Our homepage appears as a result, with the URL listed under the title and snippet.

Google is good at crawling all types of URL structures, even if they're quite complex, but spending the time to make your URLs as simple as possi bl e for both users and search engi nes can hel p.  Some webmasters try to achieve this by rewriting their dynamic URLs to static ones; while Google is fine with this, we'd like to note that this is an advanced procedure and if done incorrectly, could cause crawling issues with your site. To learn even more about good URL structure, we recommend this Webmaster Help Center page on creating Google-friendly URLs.

Use words in URLs
URLs with words that are relevant to your site's content and structure are friendlier for visitors navigating your site. Visitors remember them better and might be more willing to link to them.
Avoid:

• using lengthy URLs with unnecessary parameters and session IDs 

• choosing generic page names like "page1.html" 

• using excessive keywords like"baseball-cards-baseball-cards-baseballcards.htm"

Create a simple directory structure
Use a directory structure that organizes your content well and makes it easy for visitors to know where they're at on your site. Try using your directory structure to indicate the type of content found at that URL.
Avoid:

• having deep nesting of subdirectories like ".../dir1/dir2/dir2/dir4/dir5/dir6/page.html" 

• using directory names that have no relation to the content in them

Provide one version of a URL to reach a document
To prevent users from linking to one version of a URL and others linking to a different version (this could split the reputation of that content between the URLs), focus on using and referring to one URL in the structure and internal linking of your pages. If you do find that people are accessing the same content through multiple URLs, setting up a 301 redirect from non-preferred URLs to the dominant URL is a good solution for this. You may also use canonical URL or use the rel="canonical" link element if you cannot redirect.
Avoid:

• having pages from subdomains and the root directory access the same content    - e.g. "domain.com/page.htm" and "sub.domain.com/page.htm" 

• using odd capitalization of URLs                                                                                                         - many users expect lower-case URLs and remember them better