The pillars of salaat are five and they are what are considered fard in salaat. The person who leaves off any one of these five things whether it is done intentionally or unintentionally will render his salaat null and void.
Salaat is not accepted sitting if you have the power to stand.
Standing is fard in fard and wajib salaats, it is not fard to stand in nafil salaats. It is permissible to sit when you have the power to stand in nafil salaats.
Qiraat (recitation of Quran)
At least one ayat of Quran must be recited in salaat. If not one ayat is recited the prayer will not be valid. Qiraat is fard in the first two rakaats of a fard salaat. Qiraat is fard in every rakaat of wajib and nafil salaats. Qiraat is waived for the one praying behind an imam. It is makrooh tahrimi to recite behind the imam in prayer.
A complete bowing is one where the head and the hips of the person are equal.
Two sajdahs are fard in every rakaat of prayer. A complete prostration is one consisting of the forehead, nose, both hands, knees and feet on a hard surface. The forehead should remain stable and firm and not sink down into the surface. If the surface is soft and the head is moving (for example if you pray on a very soft foam mattress) then the prostration will not be valid.
Both the forehead and nose must touch the ground. If the nose touches but not the forehead the prostration will not be valid. If there is some wound or injury to the forehead then it would be permissible to only prostrate on the nose. The place of prostration must not be more elevated by more than ½ yard (eighteen inches or forty-five centimetres) from the place of the feet. If so then the prostration will not be valid, except where there are huge crowds, then it would be jaiz to prostrate on the back of another performing the same salaat. Also some part of your toes must be touching the ground. It is not enough to place the outer portion of the feet on the ground only.
Qaidah Aakhirah (final sitting)
This sitting must be at least equal to the time it takes to recite the tashahhud.