The Chrome Web Store allows users to search for Apps and Extensions for Google Chrome. That begs the question what is the difference between an App and Extension, is one better than the other? The short answer is neither is better than the other, they simply perform different tasks.
Is a service that adds to the content on the web page. For example, Read and Write for Google is an example of an extension because it reads the content on the page. Think of these as added value to the page. Most assistive technologies will be extensions because they help to user access information in a different way.