Our elder, father and noble shaikh, Shaikh Muhammad ibn Saalih al-Uthaymeen, rahimahullaah said:
"The types of Dhulm* are three:
1-Dhulm of the human being regarding the rights of Allaah, like ascribing a partner with Him, and that is the greatest of all dhulm.
2-Dhulm of a human being towards himself, like placing upon himself something he does not have the capacity to carry out.
3-Dhulm of a human being towards another human being, like killing a person without a legal reason. And that is the greatest type of dhulm regarding a human being"
Taken from his book entitled 'al-Fawaa'idul Muntaqaat min sharhil Kitaabit Tawheed'
*Oppression. Literally: 'exceeding the boundaries of something'