Youtube shows some more or less informative things if you rightclick on a video and choose "Stats for Nerds". Unfortunately, I could't find any documentation for the fields shown.
Depending on your configuration, it might be that it streams using MPEG Dash and it looks similar to the following screenshot. But what does the "(137/140)" stand for?
PS: in case someone is wondering: the "codecs"-string seems to be specified by RFC6381
These are itag values that refer to different video and/or audio formats. The first number corresponds to video and the second to audio. You can actually see these numbers change when you select different resolution from the player's settings.
Many but probably not all of the itags are documented in Wikipedia