@Mitch All those words are different cases of the masculine relative pronoun.
So "who/which", "whose", "to whom/which", "whom/which", "by/etc. which/whom".
There are also plural forms, and of course feminine + neuter.
The j is a Mediaeval invention, now used where Romans pronounced the i as /j/ (which is not like English j).
Note that j was used differently before modern times.
I don't know why some forms have qu and others cu. Probably has to do with the following vowel:
Labiovelars apparently disappeared before certain vowels as in Greek, but were kept before others.
Latin equus = Greek hippos, because the labiovelar changed into p in Greek before o.
A labiovelar is kʷ, gʷ, or gʷʰ.