Collectivism is the practice of regarding people in the aggregrate rather than as individuals. The collectivist believes people can be understood and judged with reference to the group that they belong to (or are supposed to belong to). The (often supposed) characteristics of the group are thought to apply to all of its members. Differences between groups are considered significant, while differences between individuals within groups, to the extent that they are acknowledged at all, are considered unimportant.
Many political systems are based on collectivism. Communism regards people with reference to their supposed economic class. Nationalism regards people with reference to their nationality. Typically, political collectivists hold that different groups have competing interests, and that the individual's interests are in fact tied up with the interests of his or her group. This line of reasoning, anti-collectivists allege, often leads to the suppression of individual rights, which are sacrificed for the alleged good of the group.