Over the past year I have heard alot about Velocity and NVelocity. Reading their documentation and doing searches on the net hasn't given me the answers I was looking for.
In what situation would I use this library in my development? What problem does it solve that didn't already have a solution?
Since the dawn of web apps, people started to think about separation of concerns in many applications, including web applications. The challenge is to separate what is view code from what is business code, or logic code. When jsps first arrived, many people where coding lots of logic in jsps directly (stuff like db access and other), breaking the basic principle of separation of concerns (jsps should be responsible for the presentation, not for the logic).
Velocity, Freemarker and others are templating engines that allows the separation of UI logic and business logic, thus facilitating changes in the presentation while minimizing the changes on the business side. These template engines have facilities for doing common UI tasks such as showing a block of html if some condition holds true, or iterating over a list while maintaining logic code outside of the view. This is fundamental for maintaining a complex application in the long run.