Lip-sync dubbing is the replacement of an on-screen speaker’s voice with the voice of another actor such that the spoken words are in relative synchronicity with the movements of the on-screen speaker’s mouth.
The goal of lip sync is to make it look like the on-screen speaker is speaking a foreign language.
“UN-style” VO – an informal term of art – implies that one hears both the original speaker at low volume and the narrator reading a translation of what he/she is saying. One often encounters this type of voice-over in documentaries, interviews, and news programming. It’s widely used where and when the intention is to preserve the character of the source language.
“Off-camera narration” is narration in which the narrator doesn’t appear on screen. He or she merely reads/speaks over the image.