Obviously the differences in the impact to teams, customers, ROI, etc. of applying the two approaches is huge and is the subject of many books and endless discussions and conferences.
But as I think more about it, I am having a hard time finding any difference between the two that doesn't ultimately map to a single root difference which is the frequency of release.
Waterfall spends time on a design, then writing the code, then testing and finally releasing. But Agile does exactly the same set of steps - its just that each one is smaller.
A key piece of the Agile approach is about learning from each release and using that to let the larger design emerge instead of trying to predict it at the beginning.
But Waterfall does this too. Its just that instead of learning every 3 or 4 weeks, the Waterfall team only learns every 6 or 9 months. But the Waterfall design still emerges. That is, waterfall release 2 will reflect what was learned in release 1. So the process is not different, its just that it executes at a different speed.
Agile focuses on close customer collaboration. But Waterfall does this too. Its just that since waterfall has a longer iteration time, an enumerated list of requirements in the form of a contract is more needed to keep everyone on the same page over the long period of time. But again, this is just an artifact of frequency. The higher the frequency of delivery, the lower the need for a contract.
Are there any other primitive differences that I am missing - or is it really just frequency?
Waterfall:
Agile:
user story) first(after each feature you completed or after the time boxed period (usually called sprint or iteration))
The difference is pretty clear to me.
With Agile, you can adapt what to build by delivering small chunk of the software frequently. You can stop when you have enough.