I came across the site Polyvore and gave it a try.
When using one of its "Create a set" feature, the site shows u a view of your items or items from somewhere else. Let's say if I open two tabs in the same browser one of which showing the items list. If I use the other tab to add a new item to my collection, the other one will automatically be updated. This only happens when I use tabs on the same browser. If I'm not wrong this technique is something called "push ajax"?
Can anyone tell me how to build something like this?
What you're looking for is probably comet. The technique is really going to come into its own when we get wide support for web sockets, until then it is kind of a hack using techniques like long polling.
However from what I can tell you're just interested in talking between two instances of a browser on a local computer. If that is the case then you might not need to go to the point of implementing push on the server you could use local storage and do rapid polling of it to look for changes. Take a look at https://stackoverflow.com/questions/1194784/which-browsers-support-html5-offline-storage