How to record your whole macOS screen, or a portion of it, with QuickTime Player
Apple has included an incredibly easy to use screen recorder with macOS High Sierra, contained inside the QuickTime Player utility. AppleInsider shows you how to use it.
The QuickTime Player lives inside the Applications folder. Open the player, and pull down the File menu. and select New Screen Recording.

A "Screen Recording" controller will pop up. If you have multiple displays, put the controller on the display that you want to record.

Click the record button. After you click the button, the QuickTime Player will display the following:

A simple click records the entirety of the screen's contents.
If you just want to record a smaller segment of the screen, for instance, a browser window, drag across the area you want to record.

The recording starts when you click the oval "Start Recording" button in the middle of the selected area.

Regardless if you record the whole screen or just a window, hit the stop button in the menubar to stop the recording.

The QuickTime Player lives inside the Applications folder. Open the player, and pull down the File menu. and select New Screen Recording.

A "Screen Recording" controller will pop up. If you have multiple displays, put the controller on the display that you want to record.

Click the record button. After you click the button, the QuickTime Player will display the following:

A simple click records the entirety of the screen's contents.
If you just want to record a smaller segment of the screen, for instance, a browser window, drag across the area you want to record.

The recording starts when you click the oval "Start Recording" button in the middle of the selected area.

Regardless if you record the whole screen or just a window, hit the stop button in the menubar to stop the recording.

Comments
But with High Sierra you can use Quick Time to record your iOS screen! Actually no, you could also do it long ago. Keep up the good work!
it is intuitive, as QT has been the default AV app in Apple forever.
If that were the case, then why aren’t there thousands of “Thank you, I never knew!” posts?
https://rogueamoeba.com/freebies/soundflower/
https://rogueamoeba.com/loopback/
https://github.com/mattingalls/Soundflower
https://github.com/mattingalls/Soundflower/releases/tag/2.0b2
When SoundFlower is installed, you can setup custom audio devices with loopback. This can be done in /Applications/Utilities/Audio MIDI Setup. You can create an aggregate device in there by clicking the plus icon and adding all the required channels e.g built-in input and output and soundflower 2ch (this allows listening plus loopback). This will show up in System Prefs > Sound as an output.
When you do a recording, switch the System Prefs > Sound to the aggregate device and in Quicktime there's a dropdown next to the record button, set that to soundflower 2ch. When you play a video like on Youtube, the audio bar will show that there's audio being picked up if it's setup ok. Then switch the output back to normal after recording as volume control doesn't work with aggregate devices.
It would be useful if Apple offered an audio loopback device, it probably wouldn't take them long to setup. It would also be useful if they put screen recording in a more visible place. Long-term Mac users accustomed to Quicktime know where it is but it's not immediately obvious. Same goes for screenshots (shift-command-3, shift-command-4, shift-command-4-plus-spacebar for window shots). A menu bar item would work ok and this can be activated in System Prefs > Displays or a separate preference called Recording/Capture and they can put audio options in there too. The menu bar can initiate a screen recording in Quicktime but it would be best to just start it and open the saved result in Quicktime when done. It should also auto-save and auto-name it when the stop button is pressed because it's not always clear that it's just in memory and if you don't save it, it will lose the recording. It would actually be good if it could save chunks while recording so that there wasn't a huge save process right at the end but that might need a new file format (file stream).
So count me thankful for the heads-up.
Wow, thanks for that, I never noticed.