CHROME EXTENSIONS
General knowledge on Chrome extensions.

Browsers use the generally same extension structure.

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An extension has a manifest, describing itself in english, its code, and 
required permissions.

The manifest explains what html file to load when clicking the "action" icon
button, or clicking options. The html file then loads js.

An extension has a special "chrome" object to hook into the extension life
cycle. For example:

chrome.runtime.onInstalled.addListener

and

// Which requires the "storage" permission.
chrome.storage.sync.set

The chrome object is available in an action's javascript too. Essentially any
context which is running under the extension.

Some other interesting methods:

// Get tabs
chrome.tabs.query

// Execute a script within the context of a page, can use a tab returned from
// above.
chrome.scripting.executeScript

Again to use all of these, you'd need the following permissions in the manifest:

"permissions": ["storage", "activeTab", "scripting"]

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The options and action pages are different.