Custom Forms allow you to add your own HTML code into your project to give you additional flexibility. If you want to show more fields and selections other than the standard text description, then the Custom Form would be suitable for this.
You can also add Custom Fields to your project. They are drop down fields you can add to your project. Once this is configured in the Settings page, this will then appear in your project on the right hand column. What's cool is that you can run a report against your Custom Fields selection!
Watch these features in action here!