What would you say are the most common mistakes about what it is?
1. That Catholics worship Mary (we offer hyperdulia, not latria, and in English the word "worship" has drifted to mean exclusively latria).
2. That Catholics deleted the commandment about graven images from the bible (we did not alter the bible whatsoever, and in our teaching materials we consolidated the first and second commandments because they are talking about the same thing, and we separate women out from property in the commandment against coveting to affirm her humanity).
3. That Catholics believe we are saved by works rather than faith (we affirm St. James when he teaches that we are saved by the kind of faith that also produces works).
4. That Catholics added seven books to the bible (which is a neat trick, considering that the Eastern Orthodox have those same books in their bible).
5. That Catholics hold oral tradition over scripture (we hold Sacred Tradition (2 Thess 2:15), Sacred Scripture, and the Magisterium (the bishops speaking with one voice) to be three branches of the same deposit of faith delivered once to the saints).