When you pray or perform Salat there are many things you have to consider such as ablution, timings, niyat or intention and so on. It is an obligation or duty of every adult and sensible Muslim and it is supererogatory.

On the other hand, Dhikr Allah is purely for those who seek pleasure in the remembrance of Allah Subhana wa Tallah. Dhikr Allah gives comfort to the heart of contemplator and it is obligatory for those who are in love with Allah Subhana wa Tallah. There is no certain posture or any limit to doing the Dhikr. You only have to have intentions to please your Lord, Allah Subhana wa Tallah when performing Dhikr.

You can do dhikr when you are alone and there is no compulsion of ablution when dhikr is performed in heart.  Allah Subhana wa Tallah says in Holy Quran “Lo! In the creation of the Heavens and the earth and in the night and day are tokens (of His sovereignty) for men of understanding, such as remember Allah, in standing, sitting, and reclining”.  [Qur'an 3:190-191]

Difference between salat and dhikr Allah is that they are both performed to please Allah Subhana wa Tallah but one is an obligation and the other is optional. After performing and before salat it is mandatory for the Momin or believer to contemplate and keep on remembering Allah Subhana wa Tallah in his or her heart. Whether you are alone or assembled in groups you can remember the Lord, Allah Subhana wa Tallah. Dhikr Allah can be comprised of first kalima `La ilaha illa Allah,’ or you can do Dhikr by saying `Allah and Asllah Hu’.