Basic communication between a Background Page and a Content Script inside a Chrome Extension.

communication.js

// In Background.js
chrome.tabs.sendMessage(tab.id, {content: "message"}, function(response) {
 	if(response) {
 		//We do something
 	}
});

// In ContentScript.js
chrome.extension.onMessage.addListener(function(request, sender, sendResponse) {  
	if(request.content) {
		sendResponse({content: "response message"});
		return true; // This is required by a Chrome Extension
	}
})