Voice dubbing, also known as ADR, Dubbing, or Voice-Over, entails replacing the original dialog in a video with the translated foreign language. As you can imagine this is quite a complex process which includes not only translating the dialog but also adapting it to match the lip movement of the on-screen actor and then recording the translation using professional and experienced native voice actors to reproduce the original on-screen performance.
Voice dubbing can be divided into the following categories:
Lip-Sync Dubbing (or ADR)
This is the most common method of dubbing for movies or TV shows. Basically this process completely replaces the original dialog with the corresponding foreign language, with all its acting and sync. Several voice actors are cast for such a production since each main on-screen character will have a dedicated voice actor assigned for the parts, so as to create the illusion that the on-screen actor is speaking in a foreign language.
U.N. Style Dubbing
You commonly see this method used in English speaking documentaries when foreign people are interviewed: The original dialog is left in the video at a lower volume and the translation is mixed on top of it. Usually the original dialog is heard for a fraction of a second at full volume at the beginning of a sentence and then it is lowered when the translation recording starts. Syncing is less critical in such scenario and as long as you start and end with the original dialog lip-syncing is irrelevant. Since we the original dialog is left in there, usually no more than two voice actors are used in such productions, one for male and one for female voices. This is a more cost-effective method of dubbing for a project that does not require the full cost of the lip-sync method.
This is the simplest method and it usually requires one or two voice actors. This method is used on videos where there are no on-screen actors and the dialog is all narrated by off-screen voices. In this case the translation is simply recorded loosely following the original recording.