Remember those times when you used Internet Explorer 5 or 6 and you used Google's toolbar to search the web? You could have installed it yourself, or it came bundled with another program and you missed an important tick mark during installation. Those unwanted toolbars still exist and Google has decided to put a hold on those silent installs.
Starting with Google Chrome 25, programs can no longer silently install extensions without your permission. They will be disabled by default, however Chrome will tell you when it detects a new extension you didn't install yourself by displaying a badge on the menu button. Then you can click through the menu to enable or remove the new extension.
Google goes a step further by also disabling previously installed silent extensions. You will receive a one-time promt showing you a list of silent extensions so you can review all installed extensions and delete some if you want to.
Google advises software developers to notify users when installing extensions by pointing them to a website and using inline installation. That way, the user knows what is installed and the extensions are hosted in the Chrome Web Store.
Source: The Chromium Blog | Images via Google