To capture what just happened:

Below are the steps to Capture moments via video and screenshots on your xbox one. Microsoft has made this process fairly easy so lets go through the steps quickly.

1. Start your game and play as usual.
2. When something cool happens and you want to record it, press the xbox button to open the guide, then press the x button to grab the last 30 seconds.

To Capture A Screenshot Instead, press Y.

Using Cortana: You can use cortana to take actions in a game by saying “Hey Cortana, Record that!“. You can also say “Xbox, record that!” to capture what is about to happen for recording a clip moving forward from that point.

Note: The screen shot is taken when you press the Xbox button. Pressing Y to save it.