These are average lightweight index cards. They are printed on the thin index paper, not the heavy stuff, so a Sharpie bleeds through. They are lined so they're not really suitable for double-sided flash cards to use with smaller children because the lines can be distracting from the information printed on the card but they're fine for taking notes or using for more detailed flash cards.
They are also good for journaling or daily chore lists. The smooth index paper is excellent for fountain pens or pigment markers. Thicker markers will definitely bleed through but fine nibs won't. I can journal on the ruled cards and doodle or make grids, etc.. Just test your pens first to avoid surprises.
The differnt colors make it easy to color code if I plan on using them for different subjects or purposes.