Its open only for Indian or Nepalese citizens. Its not open for citizens of other countries.
This is Nepalese link which says Indian citizen entering Nepal should have proper identity to prove that they are valid Indian citizens, that means no need of visa.
And this is link from Indian Embassy in Nepal. On first line it says "Except Bhutan & Nepal Nationals all foreign nationals including children MUST obtain a visa to travel to India".
Hope this helps
And if you are travelling on a car or bikes or any means of transport from one country to another between India and Nepal, you have to make an entry at border check point but its always wise to get a letter(like No objection Letter for a car, bike) from respective embassy from current country.