Google is always integrating new features into their products. Since Google operates on a web interface, not a program installed on your computer, you will automatically stay updated without having to perform upgrades. If the change is important, a message will appear within Google (for example, on top of your calendar) informing you of the new feature. However, many small features might appear without notification.
If you'd like to stay up to date on new features, check out the items below. These come directly from Google's official blogs: