The difference between pillars (essential parts) and obligatory parts of prayer
Question

                What is the difference between pillars (essential parts) and obligatory parts of prayer?.
Praise be to Allah.
    Firstly: 

    The difference between pillars (essential parts) and 
    obligatory parts of prayer is that the pillar or essential part (rukn) 
    cannot be omitted deliberately, by mistake or out of ignorance, rather it 
    must be done. 

    The obligatory part (waajib) may be omitted due to 
    ignorance or forgetfulness, and it may be compensated for by doing the 
    prostration of forgetfulness (sujood al-sahw). 
There are fourteen pillars or essential parts 
    of the prayer, and eight obligatory parts. 
They have been explained in the answer to 
    question no. 65847.