I read somewhere that authors sometimes use cards to write down information about the characters in their stories. I assume these were small cards so what was written them had to be concise. What sort of details did they write on them? Something about their appearance or their psychology/beliefs?
As much as you need to understand the character. This might be as simple as you want, like just the name, physical characteristics(small and thin), and a rough psychological profile(like character loses temper often and is controlling).
On the other hand, I've heard of authors go down to excruciating detail, like what their sun sign is, what their quirks and hobbies are, a complete Freudian psychoanalysis of the main characters , and which brand of breakfast cereal they love :) .
Unless you are writing a literary novel, you don't need this much detail. Maybe a paragraph or two for all the minor characters, and one page for the main character.
I made my own character cards. I simply wrote down everything important about the character so that i wouldn't mess it up later in the book. name, age, eye color, hair color, beliefs, family, height, weight, relationships to other characters, and then I had a misc. section. that is just for anything you think is important but doesn't really have a category.
The term "card" is fungible. Your card can be a 36x48 poster or seventeen pages in Scrivener or an entire notebook of thoughts. It could be a character quiz, a playlist, a drawing, a list of favorite books, photos of actors, bullet points, quotes, or all of the above.
Whatever helps you as the author to get a handle on your character so you know how the character will behave in the situation you are creating is what you put on your card. And not all the things on the card have to go into the story, either.
I'm not sure if this answers your question, but are you talking about a system like this one?
I also found this resource about outlining a Novel that I think would be good to use.
They are two resources I looked at in the past when I wanted to know how best to outline a novel. I understand your question, because, while Standback is correct in saying that every person is different and every bit of importance in the book is different, there are good and bad methods to writing a story, and learning about those good and bad methods might help you find one that works best to you.
For my epic fantasy novel I used index cards to identify some of the key characteristics of most of the characters. This helped me whenever I had to reference a character that I hadn't used in a couple of chapters. I also laid out the cards in a timeline to help keep track of when the charcaters were introduced and what they were doing at the time. How you use them will really depend on what you need to accomplish with them.