The Du’a (supplication) is the act of speaking to Allah (swt) while the Ziyarat (visitation) is the act of speaking with the role-models and true examples of the faith sent by Allah (swt) .
Nevertheless, Ziyarat itself is also a form of speaking with Allah (swt) since the individuals being addressed are the Prophets (as) and A’immah (as) who are individuals that invite humanity to the worship of the One True God and to struggle against the internal and external enemies. Thus in reality, the Ziyarat is nothing more than speaking to Allah through the intermediaries which He himself has appointed in order for us to reach to perfection.
The Du’a is the act of travelling towards Allah-consciousness and self-knowledge while the aim of the Ziyarat is knowledge of the Imam (as) – and in actuality this is the same as having knowledge of Allah and knowledge of ourselves.
Within the Du’a taught by the Ahlul Bayt (as), there are four main points seen: shedding of tears and showing grief, expressing one’s needs, gaining a better understanding of the faith and beliefs and the invitation to stand up and fight against the enemies - internal and external.
This is the same thing which can be seen in the Ziyarat as well, however at the same time, we are calling upon and remembering those personalities who are the best and most perfect examples of these four characteristics.
By this we mean that the Ahlul Bayt (as) are the ones whose tears which they shed in their prayers are the most sincere drops of grief; they are best able to express their needs to their Creator in the most eloquent of ways; their understanding of the faith was the most complete and perfect, and lastly, they were the ultimate manifestation of those who stood up and fought against both their internal enemies and the external despots – each one in his own way and method.
Of all the Ziyarats we have, the best example in which these four points are manifest is that of Imam Husayn (as) – especially since he was the supreme expression of rising up for Allah (swt) and standing up in the face of oppression and tyranny.