In waterfall projects, business analysts write comprehensive requirement documents and then hand them over to developers and testers. Developers will create the application according to all of the details specified in the document, and testers use these documents to create test cases. Once the code is complete and integration is done, testers will do quality assurance and developers will fix bugs before the customer performs the acceptance test.
Since the required documents need to include all of the details, it usually takes months for requirement development. And if the schedule is tight, requirements might not be accurate, or worse, not documented. Or later during the implementation, customers might change their mind and some features may have to be discarded even though they have already been built.
There are many other kinds of risks...
