Decriminalization is when the classification of an act as a crime is changed so that the act is no longer considered a crime. An example of this would be when the possession of certain amounts of marijuana was decriminalized in parts of the United States. Once the law is officially changed, then a person cannot face criminal charges for the offense.
As time goes on, society occasionally changes its mind about certain laws. It could be because views have changed or because developments have been made in the area. However, at any rate, certain acts can go from being viewed as a crime to not being viewed as a crime. When the laws are altered to reflect these changes, it is called decriminalization. Examples of decriminalized offenses include certain sexual acts between two consenting adults and old racial laws.