Most people will want to stream their video live to Twitter but you don't have to go this route. Periscope doesn't tell you anything about YouTube (because that website is owned by competitor Google) but if you want to save the video file to your PC and then load it later to YouTube you can do that at the end of the livestream even if you choose to go live to Twitter first. At the end of the recording Periscope will ask you if you want to save the video file to your iPhone's camera roll. If you click on this option the file will end up on your phone and you can just email it to yourself then load it to your YouTube channel at a later time.
Getting back to the interface there's of course the livestream to Twitter route. This will be the route most people will want to use because it's the method with the closest socmed integration.
It works like this. When you click on the broadcast icon at the start screen (there are four icons at the bottom of the screen, and broadcast is number three from the left) you'll get the message "Initializing video stream". Note however please the tiny Twitter icon located on the right-hand side of the screen in the centre. If you want to stream live to Twitter this icon must be highlighted. But while the video stream is initializing you might not be able to get the Twitter icon to turn white straight away. It takes a bit of time (Periscope has a few of these "lags" in it, including the initialisation procedure itself). After a while it'll let you switch it to highlighted and a little message appears telling you "Twitter post ON", for example. Keep in mind also that the Twitter icon adopts the same setting as the most-recent previous recording, so if your last recording had it ON then the next recording will also default to ON.
You can then use the keypad to type a message to appear with your tweet that Periscope will automatically send to Twitter after you start the broadcast.
When the big red "Start broadcast button" appears you can touch it and livestream whatever you want, whatever is in front of you or whatever is happening in your environs. People will start to watch and the little counter will show how many people are currently watching your feed. Their comments will appear at the bottom of the screen. When you want to finish recording just swipe the screen down and hit the Stop button.
Watch the save to camera roll method though because if for some reason the saving to the camera roll fails inside Periscope then you'll never get the opportunity to save the video file again. It will sit on your phone but you won't be able to do anything with it except show it to friends on your phone or view it on your phone yourself. This appears to me to be a big minus in Periscope's interface and I would suggest to developers that they look at ways to make it easy - if that initial download fails for whatever reason - to get the video file off the phone to somewhere else where it can be then used.