6 million Jewish people were killed by the Nazis in the Holocaust during the Second World War.
Holocaust Memorial Day remembers everyone who lost their lives under the Nazi rule , including millions of people killed in genocides in Cambodia, Rwanda, Bosnia and Darfur.
The theme of this year's Holocaust Memorial Day is "One Day," which encourages people to pick one day in history on which an act of genocide was committed and learn about that day.
The Holocaust Memorial Day Trust explained that they mark the day with hope for a future with no genocide. The trust said: "We learn more about the past, we empathise with others today, and we take action for a better future."
When is Holocaust Memorial Day?
Many countries have instituted their own Holocaust Memorial Days. In the UK, National Holocaust Day is marked every year on January 27.
The date was chosen because it was on this day in 1945 that the Auschwitz concentration camp was liberated by the Soviet Union.
It's also the date chosen for the International Holocaust Remembrance Day as well as some other national Holocaust Memorial Days.
At least 1.1 million people are thought to have been at Auschwitz, in Poland, which was the largest Nazi death camp.
It consisted of more than 40 concentration and extermination camps, but today a museum and memorial can be found at the site to remember "the martyrdom of the Polish nation and other nations in Oswiecim".
How is Holocaust Memorial Day marked?
In the UK. there is a national event which brings together survivors, their families, celebrities and members of the Royal Family to mark the day. The memorial ceremony can be watched live on the Holocaust Memorial Day Trust’s website.
This year, portraits of seven Holocaust survivors have been commissioned by Prince Charles to commemorate the day.
The Holocaust Memorial Day Trust said: "The Holocaust threatened the fabric of civilisation, and genocide must still be resisted every day. Our world often feels fragile and vulnerable and we cannot be complacent. Even in the UK, prejudice and the language of hatred must be challenged by us all."
The trust said memorial day is for everyone, as thousands across the UK come together every year to "learn more about the past and take action to create a safer future," adding: "Together we bear witness for those who endured genocide, and honour the survivors and all those whose lives were changed beyond recognition."
People are also encouraged to mark the day and remember the victims of genocide by lighting a candle in their window at 8pm. You can share the pictures of your candles, using the hashtag #LightTheDarkness.
