The Facts and Opinions have rune-like symbols on them. They are easily mixed up, as Joy notes that they look very similar to each other. They apparently contain information; with Facts containing information that is always true, and Opinions containing information that is what Riley Andersen or other people think.
In Inside Out
When Joy, Sadness, and Bing Bong arrive on the Train of Thought to journey back to Headquarters, Joy accidentally knocked over crates of Facts and Opinions. She quickly tried to put them back in their proper places, but Bing Bong noted that people confuse them all the time, and did not hesitate to put most, if not all, of them into a crate marked FACTS.