Basic editing, cutting, joining:
Use a real mpeg editor like VideoRedo, Mpeg Video Wizard DVD, Tmpgenc Mpeg editor or Tmpgenc Authoring Works / Tmpgenc DVD Author. Open the DVD or the VOB files from the DVD.

Join several DVDs:
Use DVD Shrink in the reauthoring mode to join DVDs, see

Advanced editing/NLE:
Use VOB2MPG and import the MPG in your advanced editor, but not all editors supports mpg so try VOB2MPG and convert the mpg to DV-AVI, Huffyuv AVI with VirtualdubMOD or Xvid4PSP and then edit in your favorite advanced editor like Premiere Pro.