The S Pen has been one of the main differentiating features between Samsung's Galaxy S and Galaxy Note lines, and this year it's much improved.
This year's S Pen is water resistant (up to 5 meters of water for 30 minutes), and can get 30 minutes of power in just 40 seconds of charge time. More importantly, the S Pen supports several nifty features, like using it as a remote to take photos, or control music and video playback. It's also good for drawing on top of your photos or videos, making GIFs out of your videos, or writing handwritten messages. You can also use the S Pen to take notes within the Samsung Notes app, which supports various brushes, pens and colors so you can draw as well as write, and a cool feature called Screen off Memo lets you make notes right on the Note 9's display without needing to unlock the phone.
The S Pen is also good for switching back and forth between apps, zooming into photos or maps, or even translating words or converting currencies. Since the Galaxy S9 doesn't support the S Pen, all of these features are exclusive to the Galaxy Note series.