Crimes against humanity are crimes that are committed against a large group of people who have not done anything wrong.  Groups who commit crimes against humanity do not hurt just one person, or just a few people.  They want to hurt an entire group of people that they do not like.  For example, in Nazi Germany during The Holocaust, the Nazis tried to kill all of the Jewish people in Europe.  This is an example of a crime against humanity.

Unlike war crimes, crimes against humanity can happen during peace or war.

Definition
Not every crime committed against a big group is a crime against humanity.  To be crimes against humanity, crimes must be:
 Planned by a government, or
 Accepted by the government.  Even though the crimes are very common, the government does not do anything to try to stop them.  In this way, the government is giving its approval for the crimes, without actually telling people to do them.

Crimes against humanity must also be committed against a civilian population (people who are not soldiers).

Examples 
Today, crimes against humanity can include:

However, in the past, not all of these things were thought of as crimes against humanity.  For example, crimes against humanity were first listed in 1945.  However, it was not until 2002 that many forms of sexual abuse (like sexual slavery) were included as crimes against humanity.pp. 8–10

History

The Armenian Genocide

The words "crime against humanity" were first used after the Armenian Genocide.  In 1915, the government of the Ottoman Empire killed up to 1.5 million Armenian people.  The government killed men or made them do forced labor.  They also forced Armenian women and children to leave their homes.  On the way, the Ottoman military raped, robbed, and killed civilians.

On May 24, 1915, Britain, France, and Russia accused the Ottoman Empire of committing "a crime against humanity."

The Nuremberg Trials

After World War II ended, many people who participated in The Holocaust were captured and put on trial at the Nuremberg Trials.  However, the people running the Trials had to figure out how to deal with the crimes these people had committed during the Holocaust.  At that time, courts could try people for war crimes.  But the courts had never seen a case where a government committed such awful crimes against its own citizens.

To allow the court to charge the Nazis for these crimes, the court passed a law.  This law said that the court could put people on trial for crimes against humanity as well as war crimes.  It defined "crimes against humanity" as:

Apartheid

Apartheid is a system of keeping one race separate from another.  One race has all the power, and the other race has none.  This continues through racist laws and attitudes.

South Africa practiced apartheid for decades.  The government passed laws saying that different races could not live in the same areas, have sex, get married, go to school together, go to the same beaches, or even go to the same hospitals.  The schools, hospitals, and other places where non-white people were allowed to go were much worse than the places for white people.

In 1976, the United Nations General Assembly ruled that apartheid is a crime against humanity.  Part of its ruling said:
Considering ... that all human beings are born free and equal in dignity and rights and that everyone is entitled to all ... rights and freedoms ... [we] declare that apartheid is a crime against humanity. 

No one has ever been tried for apartheid-related crimes against humanity.

Rape and sexual violence
The International Criminal Court (ICC) opened in 2002.  Its job is to look into war crimes and crimes against humanity.  If possible, it tries and punishes people for these crimes.  When the ICC was formed, it created a new list of crimes against humanity.  This was the first time that any type of sexual abuse, other than rape, was included as a crime against humanity.  The ICC includes all of these as crimes against humanity:pp. 8–10
 Rape
 Sexual slavery
 Making women work as prostitutes
 Forcing women to get pregnant
 Forcing women to get sterilized
 Sexual violence (other types of sexual abuse and sexual assault)

The ICC also includes all of these crimes as war crimes, if they happen as part of a war.pp. 8–10

In 2008, the United Nations Security Council ruled that “rape and other forms of sexual violence can [be] war crimes, crimes against humanity or a [part of] genocide.”

References

Other websites
 Elements of Crimes by the International Criminal Court (a list and explanation of every crime the ICC considers a crime against humanity or a war crime)

 
International law
War crimes
Genocide
Human rights abuses
Torture