Above is a field of ceramic poppies which formed part of the art installation 'Blood Swept Lands and Seas of Red' by artist Paul Cummins at the Tower of London.
Remembrance Day (sometimes known informally as Poppy Day owing to the tradition of the remembrance poppy) is a memorial day observed in Commonwealth member states since the end of the First World War to remember the members of their armed forces who have died in the line of duty. Following a tradition inaugurated by King George V in 1919, the day is also marked by war remembrances in many non-Commonwealth countries. Remembrance Day is observed on 11 November in most countries to recall the end of hostilities of First World War on that date in 1918. Hostilities formally ended "at the 11th hour of the 11th day of the 11th month", in accordance with the armistice signed by representatives of Germany and the Entente between 5:12 and 5:20 that morning. ("At the 11th hour" refers to the passing of the 11th hour, or 11:00 am.) The First World War officially ended with the signing of the Treaty of Versailles on 28 June 1919.
The tradition of Remembrance Day evolved out of Armistice Day. The initial Armistice Day was observed at Buckingham Palace, commencing with King George V hosting a "Banquet in Honour of the President of the French Republic" during the evening hours of 10 November 1919. The first official Armistice Day was subsequently held on the grounds of Buckingham Palace the following morning. During the Second World War, many countries changed the name of the holiday. Member states of the Commonwealth of Nations adopted Remembrance Day, while the US chose Veterans Day.Wikipedia)
Many people are now also wearing a white poppy as a sign of peace.