What it does is that allows you to set a reminder in the future to do something - at given time and date: buy X, call Y, do Z. Nothing to earth-shattering about that.
What is neat is the way how you do it. Actually, many ways:
1) old fashioned: use web site, enter reminder. Just type text and reQall will extract both activity (buy / call etc) as well as date and place it in appropriate category. And when time comes you will be notified: either via email, or via SMS or even by some of the IM services. Currently supported are Yahoo, Google Talk, Jabber and AIM.
2) modern: use iPhone app to type in reminder. Same goodies as wit web, only - unlike Web - you have the phone with you available quickly at the moment when you actually need to remember something.
3) modern AND cool: use the same iPhone app, and speak up to 30 second message. It will be converted to text and then analyzed same way as typed text. I am not sure whether it is automated translation or some poor fellas in third world country and listening and typing it in. The understanding is VERY good. It even handles non-native speakers of English with strong accents (like myself) with surprising success rate. I had occasionally some question marks indicating that Person's name or local place was not understood. For this occasion, you can always listen to your own audio ;-)
You can also subscribe to daily jots that sums up your upcoming events.
Best of all: it is a free service. There is a pro version for $2.99 a month which adds some nice features. I am seriously considering to go for Pro just to make sure they can survive - as long as I find that I use it more than 1-2 times a week, I am in.
Give it a try, it may be worth one small chai latte a month.