The Edshelf site describes it thus:
Using a simple and speedy ‘drag n drop’ creation tool, educators create and manage ‘Kahoots’ in the form of quizzes, surveys or polls related to the topics they're teaching; either asking quick questions ‘on the go’ to get feedback or opinion, or more in depth questions for formative assessment. Content can be shared with learners, colleagues or fellow educators globally.
Educators launch their Kahoots on the screen at the front of the class to their learners, who join through their personal devices. In real-time and with gaming elements to increase engagement and motivation, learners answer questions through their personal devices. Educators get an overview of the current knowledge levels of everyone in the room, and can adapt their teaching accordingly.
I had a go with the multi-choice quiz function. You create a free account and can immediately create your quiz. Each question can have four answers. You can add pictures to make your quiz more attractive. You can set a time limit for each question, from 5 seconds to two minutes. Questions can get points if you choose this option. Once it is finished, you can then get students to sign up and take part in the quiz.
The result is colourful and bold, well suited to mobile devices, accompanied by some music to add drama.
On the plus side, it is free and you can create a quiz very quickly, making it available to a class who can compete on it very easily. You can also tailor exercises to the abilities and tastes of your class. The competitive elee element will no doubt appeal to some classes and teachers. But we are, when it comes down to it, talking about a multi-choice test here. For linguists, the Hot Potato package of tasks provides more sophistication and range, even if not every game works perfectly on an iPad. A pay site such as Textivate may be less flashy, but offers a far greater variety of authoring options. Taskmagic 3, although a significant investment, has a much graeter "fun" factor, if that is what you are looking for.
I have to say that my first reaction is that this is a triumph of style over substance. If the teacher can construct clever quizzes, then it can be worthwhile.