The Google Chrome browser doesn't always close completely when you exit out of the last Chrome window. Sometimes -- depending on the add-ons and extensions you have installed -- the browser continues to run in the background, popping up notifications and allowing people in Google Hangouts to chat with you.
You might think this is pretty handy; maybe you want to get Facebook notifications when you have no browser windows open. Or you might think this is annoying, in which case, here's how you can fix it:

1. Open up Google Chrome and check your taskbar. Find the small Chrome icon and click it.
2. A menu will pop up. Find Let Google Chrome run in the background and click it to uncheck it.

To turn it back on, you'll need to go into Google Chrome's settings.

1. Open up Google Chrome and click the menu icon (three bars) in the upper right corner of the window. Choose Settings from the drop-down menu.
2. Go to the bottom of the screen and click Show advanced settings...
3. Under System, check the box next to Continue running background apps when Google Chrome is closed to reactivate this feature.