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

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.