Mnemonics are a psychological trick where you remember something by associating it with something novel, which your brain expects to do. The more associations (and the stranger), the better you can remember it. They're named after Mnemosyne, the Greek goddess of memory (and apparently, arbitrary spelling. Pronounced "Nem-o-seen.")
Okay, so what kind of stars exist? Oh be a fine girl, kiss me. Ouch. Hey, I'm not literally hitting on you, it's the mnemonic. Lists their orders from hottest to coldest. Our sun's a G. Specifically, G2V, a little cooler than the average G, main sequence.
Okay, music. Those lines stand for notes, but which ones? Well, in the treble clef, "every good boy does fine." In the base clef, "Grizzly bears don't fly airplanes." Oh, and sharps appear in a particular order too: "Father Charles goes down and ends battles." (Flats are in the opposite order, if you'd care to write a mnemonic for that.)
In chemistry, equations always obey OIL-RIG. That is, oxidation is loss (of electrons), reduction is gain. Redox!
Mechanics. Which way do I turn the screw again? Righty tighty. So therefore, lefty loosy.
Electronics? They're color coded: "Bright Boys Rave Over Young Girls But Veto Getting Wed."
Biology? All life is elaborately classified. "Kings Play Chess On Fine Grained Sand." (Although the last two, Genus and Species, are unique enough to identify pretty much anything in existence.)
And if this isn't circular enough, remember this poem and all your circles will be correct:
Sir, I bear a rhyme excelling
In mystic force, and magic spelling
Celestial sprites elucidate
All my own striving can't relate
Or locate they who can cogitate
And so finally terminate.
(Which, if you count the letters in each word, becomes Pi, to 31 digits, which is accurate enough for any dimensions of a circle the size of the universe, accurate to the width of a hydrogen atom. Trying to be more accurate than that in engineering is just being pedantic.)
Why do mnemonics work? Well, our brains are designed to keep novel and well connected information. any of these facts by themselves would just be discarded, but this gives a startling and well connected way of remembering, and any one component can bring the entire chain to mind. Also, because of a quirk of psychology, a dirty one works way better than a clean one.