Similar to Mark Zuckerberg of Facebook named as Time's person of 2010, because it changed the way of 500 millions of people live.
Chrome Extension
"Extensions are extra features and functionality that you can easily add to Google Chrome. By using extensions, you can customize Google Chrome with features you like, while keeping your browser free of things that you don't use."
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Chrome Web Store
The Chrome Web Store is a online marketplace where you can find a wide range of extensions, themes, and apps designed for the Google Chrome browser. By displaying each item on its own details page where you can read reviews and leave ratings, the store makes it easier for you to explore and try out apps.
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To me, both looks quite alike in term of search result and feature. The only different part is Chrome Extension contains all the free extensions whereas Chrome Web Store is provide a platform which similar to Apple's AppStore that allows user to download free/paid extensions.