When you upload a video to YouTube, it converts your video to allow to the user to choose the resolution. The original resolution will still be available.
Example:
If you upload a 1080p video, the user will have 144p, 240p, 360p, 480p, 720p and 1080p to choose.
Best Answer
If watching a YouTube video using the Adobe Flash Player: Right click on the video, Stats for nerds: