Earlier versions of Windows included an application called Cardfile, which made it so you could create simple index cards and shuffle through them. Cardfile was useful for keeping track of contacts or recipes, but it was of very limited use for writers.
For Windows 10, however, there's an app in the Microsoft Store that has become a regular part of my writing process. The app, called Index Cards, creates highly-customizable cards that can be used to track characters, settings, or anything else.
You can download the app for free, but to get the most out of it, I recommend shelling out the paltry $5 for the Pro version, which includes a number of useful features you'll miss out on if you opt for the free version. The free version is still useful, though. Some of the features I'll show in this post are not supported in the free version.
Cards can be created in a number of formats and styles. They can be colored, lined, blanked, or use a graph pattern. Templates exist for various list styles, a dot grid, and more.
To further help you organize your cards, you can attach colored tags to the top. You can type on the cards, or, if you have a computer that supports a pen (or fingertip), you can write on the cards, Cards are two-sided, just like the real thing, and you can use different designs for the front and back.
I can't show you how I've used Index Cards to keep track of characters without giving away more about my story than I'm willing to reveal, but these samples should give an idea of some of the things you can do. I refer to my cards all the time as I write and revise.
Here's a sample from my story, one that doesn't give away too much:
Basically, you can do just about everything you can do with actual index cards, and much more. The only thing that's difficult to do is shuffle cards. You can move them around a virtual tabletop, though, rearranging at will. You can use card and ink colors to easily organize types of characters, settings, or whatever.
Index Cards is easy to master, and it's as versatile as you'd want paper index cards to be, with the added functionality of a well-designed computer program.
One of things I really appreciate about Index Cards is how responsive the developer is. For a free/$5 app, he can't be making that much money, but he responds to user requests and updates his app frequently.
If you write on a Windows computer, and especially if you have a touch screen (but even if you don't), this app is a must-have for organizing and tracking story elements.