Salatul Witr is called Wajib, missed only if you must, is not mandatory prayer but strongly recommended conducted at home or in the mosque, alone or in a congregation. Now days , this Salat is mostly prayed in congregation during holy month of Ramadan as Part of Salat Taraweh, this salat is performed as the last three Rakaats.
Time of this Special Prayer
This is permissible to be conducted only during night after last Mandatory Prayer which is called Isha Salah and continue to pray this salah any time until before Fajr Salah. Most scholar recommend to be prayed before sleeping others recommend to be prayed after sleeping when you wake up for Fajr Salah . so before Fajr Salah Adhan, as soon as fajr salah goes , that is the end of prescribed time for this salatul Witr.
Witr is an Arabic word that means odd number, so Salatul Witri simply means odd number Minimum one and miximum is 11 Rakaat.