According to Susan Moskwa, Webmaster Trends Analyst, Google Webmaster Central Blog: “URLs are like the bridges between your website and a search engine's crawler.  Crawlers need to be able to find and cross those bridges (i.e., find and crawl your URLs) in order to get to your site's content”. 

If your URLs are complicated or redundant, crawlers are going to spend time tracing and retracing their steps. However, if your URLs are organized and lead directly to distinct content, crawlers can spend their time accessing your content rather than crawling through empty pages, or crawling the same content over and over via different URLs.

Susan provides exceptional learning in a slideshare that can be accessed here.

More tips:

Length of URLs - keep your URLs as short as possible and try to remove all unnecessary folder names. The shorter the URL, the higher the keyword density and the better for your placement in search engine results. Likewise, the closer your main keywords are to the beginning of the URL, the better.

Keywords in URLs - use the same main keywords your webpage is optimized for as this should be the search phrase you want to rank high on. Expanding the range of keywords should not be done in an URL, this should be done in <h2> or <h3> tags or plain text.

Use Hyphens - try not to use underscores in URLs as they are not used as dividers by Google. Use hyphens, periods, or commas instead. Hyphens or dashes are more user friendly and people will find your URL easier to remember. If you use underscore, you will actually have to search on “keyword1_keyword2” to find your webpage. Using hyphens will allow your pages to be found using “keyword1-keyword2” etc. 

Best

Denice MacDonald