Armistice Day or Remembrance Day
It marks the day World War I ended, at 11am on the 11th day of the 11th month, in 1918.
The anniversary is used to remember all the people who have died in wars - not just World War I.
This includes World War II, the Falklands War, the Gulf War, and conflicts in Afghanistan and Iraq.
A two-minute silence is held at 11am on Remembrance Sunday (the Sunday before Remembrance Day).
The first two-minute silence in Britain was held on 11 November 1919, when King George V asked the public to observe a silence at 11am. On Remembrance Sunday, HRH The Queen, members of The Royal Family, top politicians attend a big ceremony at The Cenotaph in Whitehall, London (this is very near to The Houses of Parliament, Downing Street where the Prime Minister lives and Buckingham Palace). This memorial service is always shown on live television in the UK.
Wearing a poppy was inspired by the fields of poppies that grew where many of the battles were fought.
They say that the red poppy represents remembrance and hope.
But there are also other coloured poppies that have different meanings.
The purple poppy is often worn to remember animals that have been victims of war. Animals like horses, dogs and pigeons were often drafted into the war effort, and those that wear the purple poppy feel their service should be seen as equal to that of human service.
Some people feel that the red poppy glorifies war and conflict. Instead they might choose to wear a white poppy.
The white poppy is handed out by a charity called Peace Pledge Union, which promotes peace.
They say that the white poppy commemorates people who died in conflict, but focuses on achieving peace and challenging the way we look at war.
An animated short film about the journey of the poppy and what it means
Leonard Cohen recites “In Flanders Fields” by John McCrae