There are 11 things wajib in the prayers:
1. Niyyah: Intention to pray for being near to God.
2. Takbirat-ul-Ihram (Allahu Akbar after niyyah).
3. Qiyam: To stand.
5. Two Sajdahs.
6. Qira'at: to recite Surah al-Hamd and another Surah in first two rak’ats and Tasbihat al-Arba’ah.
(Subhanallahi wal-hamdu lillahi wa la ilaha illal-lahu wallahu Akbar) or Surah al-Hamd in
the 3rd and 4th rak’ats.
7. Dhikr: to recite tasbih in ruku’ and sajdah, as will be explained later.
10. Tartib: To pray in the sequence prescribed by the shari’ah.
11. Muwalaat: To pray without interruption or gap.
Out of the above mentioned wajib acts of prayer, 5 are rukn and the others are ghayr rukn.
Rukn means such actions which invalidate the prayer if they are left out or added, though unintentionally.
These are (1) Niyyah, (2) Takbiratul-Ihram (3) Qiyam at the time of Takbiratul-Ihram and just before going to Ruku, (4) Ruku and (5) Both sajdahs together. If any of these are left out, or added whether intentionally or unintentionally, the Salat will become batil (void).
Ghayr rukn means such wajib acts which invalidate the prayer if they are left out, or added, intentionally. But they do not invalidate the prayer if they are left or added unintentionally. These are the remaining wajib acts of prayer.