The following Prayers are regarded as Wajib (necessary) Prayers:
Three Rak‘at of Vitr.
Two Rak‘at of ‘Idul-Fitr and two Rak‘at of ‘Idul-Adha.
Two Rak‘at offered while performing the Tawaf of the Ka‘bah.
If a person misses these Prayers intentionally, he is deemed to have committed a sin. However, if he misses a Wajib Prayer unintentionally, e.g., through forgetfulness, he is not required to offer it as a Qada Prayer. Qada means offering of a missed Prayer.