We're almost at the end of the alphabet, with the somewhat challenging 'X'. There isn't a great deal to say about the XLR connector, except that it is an interesting exampe of how professional equipment differs from consumer equipment.
The first thing one notices about an XLR as compared to, say, the RCA plugs in the back of your stereo is that it locks into place with a satisying click. This does two things: first, it makes sure the connector stays in place. This is especially sensible in a permanent installation where there's no reason to remove it and you don't want people to have to re-connect and disconnect. Second is that there are three pins instead of the two you might have been expecting. (One professional tells a story about an applicant for a technical position who identified the three pins as "Left, Right, and Ground." To a professional, this is quite thoroughly and obviously wrong). It's called a balanced audio signal in that there's the signal, a ground, and an inverse of the original signal. Balanced audio is used to run longer distances than unbalanced because when the negative signal is flipped over and added to the positive, any noise should cancel out to some extent. This leaves you with a much cleaner signal than you'd get with unbalanced audio. XLR connectors can also be easily field-terminated by soldering (as can RCAs).
As an aside, if someone ever asks you how to wire an XLR, just remember this rhyme: "Two is hot, three is not." Pin 2 is +, Three is -, and one is the shield or ground.
With the growing trend towards digital video, consumer-type connectors have been invading the commercial world in the form of the now-ubiquitous HDMI. I've seen quite a few cases in which this cause issues, especially in the case of plenum-rated HDMI cables with very wide bend-radiuses. They have a natural tendency to pull out of connectors, especially given the fact that the cable might need to be twisted to align the connector correctly. There have recently been improvements, with locking HDMI connectors starting to come to market as well as field-terminatable HDMI connectors. The latter still requires special tools, special cable, and what appears to be a fair bit of patiences. It's a kind of solution the professional world is still seeking, but with HDMI the de-facto standard digital input and output it's a solution we will certainly need. (Yes, HD-SDI is a digital format which uses locking BNC connectors and doesn't have DRM built into it. It's arguably a better professional level choice, but can't send HDCP protected content, doesn't carry the EDID device identification codes, and is primarily used in broadcast applications).
I'll see you tomorrow for Y, as we run through the home stretch.