Or it could be a writer's journal if you don't have MS Office. Nothing annoyed me more as a reader then to have something in a story change. The most common is hair or eye color. The story starts with this person having brown eyes that turned blue later in the story--colored contacts, I guess... I started using Excel early on when I got tired of retracing my steps in a story because I couldn't remember the character descriptions at a point where I needed to know it later in the story. I first started writing it down on paper but this became complicated when writing a series where characters go from few to many. I needed a logical way to not only keep track of it, but also be able to look them up again without thumbing through sloppy pages and trying to read my own hand writing.
With Excel I was able to put my characters, places and the like into logical order and easily refer back to them at any time. People I grouped by family or ships (in my sci-fi). For a ship, I would have a column for rank, then name, then for each character definition such as hair, nose, face, ears, eyes and so on. I also had a description column at the end for things I wanted to remember, like this person is related to that person, or this person was 16 at this point but is 20 at this point, and so on. Unless your memory is as sharp as a tack, it can be easy for loose track of which baby belongs to which parent and their names; trust me, I know.
A funny side note: I remember a comment I received in a writers critique group, where each person offers up a chapter to be read between sessions and then discussed at the next group meeting. My part of my story had Pete, April, her father working on a problem in trying to clear a log from a road. One within the group asked me where Barbara was at this point--Barbara being the 2-year-old daughter. She said that as a mother, she would be worried if her daughter wasn't somewhere close by. Funny, I had never thought of it from that perspective. So, I added the daughter into the scene and had a little fun with it. Pete looks over his shoulder to see Barbara sitting on the shoulder dumping handfuls of dirt over her head. He keeps quiet about it to let her have her fun, which ended when April sees her a little later...
When writing science fiction with ships and aliens and the like, there can be a lot of moving pieces to have to remember or be constantly searching back through the pages to find. Keeping them organized in Excel has saved me a lot of time. I even used a tab to map the names and movement of ships during a large space battle. Without it, I would have had trouble keeping track of which ship went where as the battle progressed. This one involved three species, so it had a lot of moving parts, ships and ship personnel going on all at once. From start to finish, I kept it all moving correctly and accurately by using the spreadsheet to keep track of it from me so I could just keep writing.
Writing a series makes the need to map reoccurring information even more important. The last thing you want to do is describe a key person in your story one way in book one and another in book three. A reader will catch that in a heartbeat, and you'll be kicking yourself for not catching it first. Being able to look up your character in your spreadsheet to refresh your memory of that character helps prevent these mistakes. You might think that you wouldn't make such a mistake, and I applaud you for having such a strong memory for details, but for us who can loose track over time, this will be like having that eidetic memory. Before I started using Excel, I can tell you I spent a lot of time thumbing back through the earlier pages of my manuscript trying to remember what color eyes this character had, or how tall they were. Not any more. I got too smart for that - even if it did take time to figure it out...