Nearly every piece of fabric has its own index card. (Exceptions are small scraps and quilting cottons.)
On this index card is written useful information, for example, the type of fabric, how much there is, where it came from and when it was acquired, and pattern ideas. A little swatch of the fabric is glued onto the card.
The cards are kept in an index card box with little dividers. The dividers are labeled according to fabric type, for example, fancy fabrics, knits, wovens, etc.