What do these have in common: XLR, line-level, mic-level, ADAT? They are all audio signal types. In other words, they are different signal formats that various cables can handle, and you must understand the basic difference between them to make a studio work.
So lets start at the beginning and divide them into manageable categories. We have four basic types: first there are microphone signals, then regular analog signals, digital signals, and other digital signals. We'll discuss each of them in that order.
First, the microphone signals. If you look up how microphones work, you'll understand that the diaphragm of the mic converts sound waves in the air into electrical signals that are amplified in the microphone itself. The electrical signal is very weak at this point, making it a mic level signal. An XLR cable is a specific format of cable designed to carry mic level signals. In other words, XLR cables carry mic level signals.
A mic level audio signal clocks in at about 2 millivolts, or 0.002 volts. That's fairly small, considering that most audio in a studio runs at a much higher strength of 316 millivolts, or 0.316 volts. We're still talking far less than the voltage of a AA battery, but keep in mind that the difference between the two is large. That is what mic preamps are for: to amplify a mic level signal into a line level signal.
As noted, a line level audio signal is much stronger in voltage than a mic level signal. The difference that makes to us as studio engineers is simple - it simply won't work to plug a mic level signal into a line level input. A microphone must be plugged into a mic preamp, and only then can the resulting line level signal be plugged into other audio gear.
Once you understand this basic difference between mic level and line level audio signals, it clears up a cloud of confusion over signals and formats. It's all a matter of voltage, really. But so far this all deals with analog signals, what about digital?
When we talk about digital audio signals, everything starts to become more complicated. The good news is that we as home recording engineers don't need to worry very much about it. We still need to know a little, but the work isn't that hard. Why? Because there are a lot of good audio recording interfaces that take a mic level or line level signal, convert it to digital, and send it to a computer via USB or FireWire all in one box. That means less hassle for us. Yay!
For starters, know that to get digital audio from one place to another, there are a few popular competing formats. With analog, it was simple: loud means loud, and soft means soft. But with digital, equipment must agree on a standard, like which bit gets sent first, and how the bits are organized. To ensure that we don't plug a type A cord into a type B plug, there are different plug ends.
The popular types you might see on recording gear are ADAT Toslink or ADAT Lightpipe (the same thing as ADAT Toslink), and S/PDIF (pronounced spi-diff, and stands for Sony/Philips Digital InterFace, if I remember right). The Lightpipe connector actually uses an optical cable, not a standard cable, so it is a small plug, kind of squarish with the end of the optical channel visible. The S/PDIF cable looks like a monitor cable for a computer, complete with the screws at the sides.
As long as you obey the rules and plug only same-type equipment together, you should be OK. But this is digital land, and strange things happen sometimes. In that case, read the manual, and double check your connections. :)
Actually, there is no need get worried about digital audio signal kinds. I never have to worry about it with my gear, except to plug it in the first time. Most audio recording interfaces take mic level or line level signals, and convert directly to USB or FireWire. I have a more complicated setup, but it isn’t that hard. First, my mics feed into a PresSonus DigiMax , which connects with an ADAT Lightpipe connection to my MOTU 828 audio interface. The digital signal is sent from there via a FireWire cable into the computer. No sweat.
So figuring out audio signal connections doesn't really need to be all that hard, it just takes a little basic understanding to figure things out. We covered the basics here, so you are good as gold. Which really isn’t a bad investment, they say...