Yet another concept that I wish I could properly understand: third normal form. I think it describes a database schema that's well-factored, with a place for everything and without redundancies among the various tables and records and fields. But then there are other "normal forms" --- and I have to admit that I can never hold the distinctions among them in my mind for more than a few days, at most.
For me it's just like the proof of Gödel's theorem, or conjugations and declensions in Latin, or the notions used to build the hierarchy of infinities in set theory, or the details surrounding a host of other philosophical-mathematical-linguistic ideas. I'm starting to believe that my mental stack must get corrupted when I try to push too many levels of abstraction onto it ... and the old ^z neural network retaliates by quietly discarding the buffers, resetting the pointers, and leaving me back in my usual befuddled state!
I'm not sure that this adds much but, for the sake of a link ;-) here's something I wrote back in March, 2001 when I was learning for myself about some of the differences: