A SIN can be  defined as any activity which is forbidden in your religion. To determine which activity is forbidden in your religion it is recommended you read your Holy Scripture in a language that you understand.

For example Lying is forbidden in all religions you SIN every time you lie, you SIN every time you accept a bribe. 

For Muslims it is a SIN to drink Alcohol or eat Pork. For Hindu's and Christians it is not considered a SIN. For Muslims Charging Interest on a Loan they give to a needy is Haram where as in Hindus and Christians there is no such limitation.

The concept of SIN comes from religion. Almost all religions have this concept only the Nomenclature differs For Hindu's SIN is called PAAP , for Muslims SIN is called Haram/Gunah.