We all know that Flash is a popular video player which has been in used from a long period of time, even if you take a look at most of the online video sharing platforms like YouTube, Vimeo and so on they uses Flash Player to play videos for you.
As technology is being changed and HTML5 emerged which tries to kill flash with its feature. Popular video sharing site YouTube supports both Flash and HTML5 video players for users but gives priority to flash player, if flash is not available on users machine then it try for HTML5 version. This is due to that some of the web browser wont support HTML5 yet and if YouTube play HTML5 version videos then it wont work.
If you want to use HTML5 on YouTube then you can set HTML5 as default for yourself by visiting http://www.youtube.com/htm5/ and activate it. Now HTML5 video player is out of beta phase and if you are a Chrome user then you will notice that above page will list HTML5 as your default and there is no way back to select Flash as your default one, if you access that page with some other browser then you will be able to switch back to flash.
Sometime back Google has announced that it will kill some plugins for Chrome and flash was one of them, which means at this time Chrome users cant switch back to flash video player and slowly YouTube will implement it on other browsers also and will prefer HTML5 whenever possible not the flash player.