Clipping = recording too loud, ie over 0dB. It can hapen to the mics (rarely), the pre-amp (sometimes, some of them have an "over" button), and the recorder. This is why you keep an eye on the levels for the first couple of minutes, if you can. Occasional overs (usually drums) can be OK, providing it's not causing distortion.
Brickwalling is similar to above, though I'd use it when the majority of the recording is over 0dB. It very often causes distortion. The Velvet Revolver gig I mentioned above wasn't over, but the mics couldn't handle the sound pressure / levels, and distorted.
Pops are when you have equipment issues, just try not to disturb anything while taping. Moving cables etc can cause this. Sometimes the PA will pop at you - for example when the guitarist changes his guitar, and hasn't muted his gear. In this case you'll have it on the tape, whether you want it or not.
It's possible to reduce volume in post when you have pops, or in the case of hair metal bands, explosions on stage..
Oh to be young and healthy..try stealth taping when you're about to pass out!
Humbug