A Zettelkasten is a collection of small notes, each linked to each other using some convention or system. It is a way of structuring notes/knowledge which has worked very well for me.
It is documented nicely at zettelkasten.de if you are interested in reading more.
I have found this works well for me because it seems to reflect how my mind represents ideas. Often ideas in unrelated fields end up being connected in some way. Being able to tie those ideas together easily and lay out those connections visually helps me to think about them more clearly.
Tags vs Categories describes my preference for tags over categories, and links between non-heirachical notes over structured folders of notes.