Copy mode is used with DVDs or Blu-rays when you wish to have the output in the same format as the Source, either VIDEO_TS or BDMV folder or ISO, with either identical or changed (compressed, edited, etc.) characteristics.
Ripper is used when you wish to convert a VIDEO_TS or BDMV Source to a video file format (MP4/MKV/AVI etc.)
File Converter is used when you wish to change the format or characteristics of a video file (e.g. MP4 to AVI etc. or MKV to MKV with different settings).
You can get more details for each of these modes and others on the DVDFab website.