What is a "derivative work" and when do I need ShareAlike?

A derivative work is anything you create by modifying or transforming the original — translations, adaptations, remixes, edits, mashups, dramatizations, etc. Mere format-shifting (e.g. encoding an MP3 to FLAC) usually does not count.

If the original is licensed SA (ShareAlike), and you publish your derivative, you must license the derivative under the same Creative Commons license (or a compatible one).

If the original is ND (NoDerivatives), you may use the work as-is, but you may not publicly share derivatives. Private modifications stay fine.