Some WordPress posts come with long URL. For example http://yourdomain.com/how-to-find-a-good-life-partner-in-life
As you see long addresses are bit bulky and hard to remember. You can easily make small URLs using slug. You can define your URL using slug.
When you’re writing a new post you can change the slug. In WordPress 2.5.X, it’s straight forward. See the figure given blow. Click the edit button and change the slug as you need.
In the other hand these long addresses will help in SEO, but it’s hard to remember, so it won’t bring you returning visitors. Also remember dashes will help you with SEO.