I read about Background page and Content scripts at developer.chrome.com but I'm confused with them, I cannot understand when to use background scripts and when to use content scripts. For example:

manifest.json:

{
    "name": "Hello World",
    "version": "2.0",
    "manifest_version": 2,
    "background": 
    {
        "scripts": ["background.js"]
    },
    "content_scripts":
    [
        {
            "matches": ["http://*/*", "https://*/*"],
            "js": ["js/myScript.js"]
        }
    ],
    "permissions": ["tabs", "http://*/*"],
    "browser_action":
    {
        "default_icon": "icon.png"
    }
}

If background.js is:

// Called when the user clicks on the browser action.
chrome.browserAction.onClicked.addListener(function(tab) {
  alert("test");
});

It works well, but if I put the same code above in myScript.js, it doesn't work.

So I don't know which script should be located in background.js, and which should be located in content scripts.

javascript google-chrome
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