I think you have already got answer to this question while discussing the question -" Did you ever commit crime?"
Paap means wrong doings or a sin or a crime. Everything on earth is Paap except love, prayer,helping others, saving life, development and giving birth to a child.
'Paap' is Hindu equivalent of 'sin'. This is any thing prohibited by religion. This is a sort of penal code with religious sanction. There is also provision for punishment in shape of penance or charity/ yagna as suggested by Purohits. For example, if you kill a cat, the remedy is to give in charity gold worth weight of killed cat. The charityis usually in shape of 'dakshina' to Pundit/ Purohit. If you commit any 'paap' and do not correct this by penance or charity as advised by Purohits, you will suffer in Hell or 'Narka' maby be Kumbhi Nark or some smaller one. Yama will award punishment on basis of report which is like Police F.I.R. by chitragupta, Accountant and record keeper of Yama Raj, the chief Justice of Hindu after death life.
paap is that feeling of mind in which our mind let us feel that something has went wrong which shouldn't be happen and it has affected other one badly due to us.