And where do look, if there is a standard for these thingies?
I understand that these are query strings, but who decides which keys are there and what they mean?
And if they depend completely on the server's implementation, then how do you know what the "si" key means, except from experience?
It's an individual tracking code that tells Google who generated the youtube link you click on. That way, they can see who you're talking with on other websites.