I see what you mean by a structure now. At this point, you could consider XML, but I'd probably stick with something like CSV.
As someone else mentioned, it's generally a good idea to store data format versioning in the file as well. If you go XML, it can be in a version= attribute. If you go CSV, make the first line have the col header information.
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I used to drive a Heisenbergmobile, but everyone I looked at the speedometer, I got lost.