Here is a brief all-in-one guide to summarize what we know about SEO for URLs. Here we go:
- Describe your content - Basically URL structure should roughly describe the content of your page.
- Keep it short - According to Google’s Matt Cutts, the best practices for URL Length should not be more than 3-5 words. If there are more than 5 words, Google algorithms typically will just weight those words less and just not give you as much credit.
- Don’t be Case Sensitive - Unlike URL for your domain, URL is case sensitive - meaning that if by any reason, you started with upper-case URL structure, it is recommended that you stick to a an upper-case version, remember that this can cause a few issues: people are most likely to link to the standard lower case one and you might both lose link juice and suffer from duplicate content issues.
- Hyphens Separate Best - Although Google has no individual preferences between hyphen and underscore (meaning you won’t be penalizes for either of the versions), hyphens are more preferable as Google “sees” each hyphened word as an individual word.
- Don’t be spammy - Try to avoid your URLs perceived as spam with the usage of unnecessary spam keywords.
- Use Static URL instead of dynamic URL - Googlebot crawls dynamic URLs and interpret the different parameters. It’s always advisable to use static content with static URLs as much as possible, but in cases where you decide to use dynamic content, you should give us the possibility to analyze your URL structure and not remove information by hiding parameters and making them look static.
I hope I have covered almost all aspects, and if you have something to add, please kindly leave me your comments. 
