24 fps is the standard in the united states and some other countries for syncing sound to picture. But if only it was that easy, the camera must be crystal sync to achieve perfectly synced sound. Although if you are shooting at 24 fps without crystal sync you can manage to stay in sync for roughly 25 secs,if you run the camera longer than that your sync will drift when syncing in post.
18 fps, as stated before is "the home movie standard." Sound cannot be synced to 18fps, and when 18 fps is projected at 24 fps the picture appears to be sped up (in the 24 fps world 18fps is called undercranking) All the films you watch at the theater are projected at 24 fps, if theres any fast motion in the film you are watching at the theater then it was shot at a lower frame rate (undercranked.) The opposite is overcranking= slow motion (36fps, 48fps etc.) when projected at 24 fps.
Hope this answered your question.
- Chris Gravat
Orlando, FL