Cap = capacitor, when they go bad your treble will be rolled off. A capacitor in series with a circuit is a low pass filter.
crossover = the bit usually behind the terminal plate that determines what frequency signals get sent to which drivers (where you'll find the capacitors - the inductors almost never go bad) A capacitor is a high pass filter when in series with an AC circuit. An inductor is a low pass filter. The simplest crossover I've ever seen is in my EPI 100s, it's a simple 10uF cap in series with the tweeter and the bass driver run full range and relying on mechanical roll off.
voice coil = the turns of wire on the speaker cone (you can't see it, but it's what the two tinsel leads connect to) that make it actually move when voltage is applied. Needs to be centered so it doesn't rub on the magnet. If it's not centered, you'll know... when the foam goes bad this is a possibility, and this is also the one important thing that you need to pay attention to when gluing new foams on.
Tube Clip - I meant punch up on YouTube and feed your receiver with your laptop look for a video where the audio is a e.g. 30Hz tone