In The Name of Allaah, The Most Merciful, The Bestower of Mercy.

What is Dhulm -injustice or oppression? Dzulm [injustice (or oppression)] is of three types according to the people of knowledge (i.e. the scholars):

Firstly: The greatest of them is shirk and why is shirk referred as Dzulm [injustice]? That is because the basis of injustice is to place something in other than its rightful place. And the meaning of shirk is to direct acts of worship to other than its rightful place and this is the greatest injustice – because when they (i.e. the people of shirk) directed acts of worship to other than its rightful place and to those who are not deserving of it, they equated the created (beings) to the Creator and a weak one (i.e. a created being) to Al-Qawiy [The One Perfect in Strength (i.e. Allaah)], who is not unable to do anything. Therefore, is there any greater injustice after this?

The second type of Dzulm [injustice] is committed when a person oppresses himself by committing sins. A sinner oppresses himself because he exposes himself to punishment, even though it is obligated on his to save himself and place himself in an appropriate situation and that is to obey Allaah etc

The third type of Dzulm [injustice] is committed when a person oppresses others (or commits injustice against them) by taking their wealth (unlawfully), or backbiting them, or spreading tales about them, or stealing their wealth, or violating their honour through backbiting, tale carrying, slander, or shedding their blood by way of unlawful murder, or beating them, inflicting injuries, degrading them without right etc [Paraphrased. Source: I’aanah Al-Mustafeed Bi-Sharhi Kitaab At-Tawheed’ page 55-56 by Shaykh Saaleh Al-Fawzaan  (may Allaah preserve him)]