Some basic rules:
If it has a transformer or a voice coil, it has an impedance, which is a calculation of the inductance and resistance (the old 'if a flag pole is this tall and the shadow is this long, what is the hypotenuse?' formula) (the AC part of the equation) which is a measurement in HENRYs and required a voltage and frequency. The combination of all those things is calculated to REACTANCE. But for the most part, it's referred to as IMPEDANCE. This is because the coil of a mic, speaker or transformer has a DC measurement, but with a 1kHz signal going in, the DC part of the coil and magnetic field the AC signal creates has an influence on the "AC Resistance" or inductance. For reference the impedance is generally called "Z", low Z, high Z.
If it's a newer device that has no transformers, it may still be referred to as IMPEDANCE, but it is generally just resistance.
AND FYI: NEVER leave any device hooked up to a signal path if the power is off. It won't hurt anything, but the circuitry inside the device that is off changes from a nice stable impedance to a complex collection of resistors, diodes, capacitance and inductance that will wreak havoc with the tone. I have had people call me up and say "Yesterday it sounded really good, but today the mids sounds mushy". My first question is "What is hooked to the console that is not powered on?" The answer is 99% of the time is "OH". This includes a digital interface, cassette recorder, 2 track recorder, CD recorder, DSD recorder, etc.
-Mics can have an output impedance of 25-300 ohms output
-Guitars typically 5K ohms to 20K ohms, but with the output pots they can vary fro the pickup impedance to 250K ohms
-Speakers typically have 2-32 ohms
-audio equipment with transformers 150-600 ohms input and output
-audio equipment without transformers usually 10K ohms to 50K ohms input and 5-200 ohms output
When matching things with no transformers, low Z output into high Z input.
When matching things with transformers, you have to follow the manufacturer's suggestions, as a tube device feeding into a solid state device may require a 600 ohm resistor to "terminating" the output. Loading it correctly will greatly effect the way it sounds. Same goes with some inputs.
Matching impedances generally gives you the best transfer of signal from one device into another.
Some other rules, devices that lose signal easily from any kind of load (like a mic or pickup) usually follow the "bridging load" rule, where the source is say 150 ohms so the device that it plugs into needs to be 10X that or 1500 ohms. This rule will usually only drop the signal output level of the device by a dB. This is why a mic pad is designed with so many resistors, so it loads the mic with 1500 ohms and loads the input transformer with 150 ohms. This is important for the mic and preamp to retain it's specifications. Though, when two coils (mic + transformer) are connected they also have interaction between them from reflections, etc. that can effect the sound, and when you introduce a pad, the solid resistance loads for each of them tend to "calm" the interaction between them down and changes the sound.
Another rule, what ever you do to one side of a transformer will reflect onto the other side at the ratio of the windings, so if your transformer is a 1500 to 10K transformer, the 10K side needs to be loaded with 10K so the other side is actually 150. It has a 6.6 X ratio, so if the 10K is loaded with 15K, the other side will look like 2.3K.
I'm sure someone will correct somethings, but I'm trying to simplify 50 years of learning into one page.
