What is difference between two way data binding and reactivity?

Reactivity is in fact more general than data binding. With reactivity you can implement data binding, in a really simple way, e.g.

var myAwesomeData = "some data";var myAwseomeDependency = new Tracker.Dependency();    var getData = function () {  myAwesomeDependency.depend();  return myAwesomeData;};var setData = function(value) {  if (value !== myAwesomeData) {    myAwesomeData = value;    myAwesomeDependency.changed();  }}

Now, every time the getData routine is called within a computation, so basically within Tracker.autorun environment, it gets recomputed. By default the meteor's collection API is implemented to be reactive, so every time fetch some data from you'r database you can be sure that it gets updated as soon as the data changes.

Also note, that you can use the above reactivity pattern without any database or values, so for example you can trigger and monitor events, states and so on.

This Wikipedia Article will help you:http://en.wikipedia.org/wiki/Reactive_programming

It basically says, that changes of data in specific dataLayers are automatically propagated. This paradigm seems to be the generic term and each framework with databinding / two way databinding is building on it and gives their technique a different name.

My understanding is that two-way data binding is a form of reactive programming. Reactive simply means that a flow of changes in your data drives action. Whether the change comes from both the DOM and the data in your application or just one of those, does not really matter.