At this time of year, lots of people choose to wear a poppy.
Here is Newsround's guide to poppies.
The history of the poppy
Some people wear a red poppy because they want to remember people who've sacrificed their lives or been injured during different wars around the world.
It grows wild in many fields in northern France and Belgium.
This is where some of the deadliest battles of World War One took place.
Many people died between 1914-1918.
Poppies are tough flowers, and can grow anywhere, but are also very delicate.
That is why they are thought to be a good symbol to remember those who died.
Why did people start wearing poppies?
In May 1915, a Canadian doctor, Lieutenant Colonel John McCrae, saw the fields of poppies and wrote a poem called 'In Flanders Fields'.
The poem became very famous and it inspired American Moina Michael to make and sell red poppies from a material called silk.
These poppies were brought to England by a French woman, Anna Guérin.
The charity the Royal British Legion, was set up in 1921. They ordered 9 million of the silk poppies, which were worn by people on 11 November, which is known as Remembrance Day. This was the first ever 'Poppy Appeal' and it raised over £106,000.
Today, all money raised from the Royal British Legion Poppy Appeal goes to help members of the Royal Navy, British Army, Royal Air Force, Reservists, veterans and their families.
What does the poppy mean?
The Royal British Legion is one of the main charities linked with Remembrance Day. It explains that the red poppy is a symbol of remembrance and hope.
It stresses it is not "blood" red or a sign of support for war or death.
Also, it should not be seen as a symbol of religion or politics, the charity says on its website.
When do people wear it?
There are a lot of different arguments about when you should start when your red poppy.
Some people say you should wear your poppy from 31 October.
Others say you should wear it in the 11 days leading up to Remembrance Day.
Some believe you shouldn't wear one until after Bonfire night on 5 November.
The 2015 Royal British Legion's Poppy Appeal was launched on Thursday 23 October. But they say if you want to wear a poppy, wear it anytime you like.
Does it matter which side you wear it on?
Many people say you should wear it on the left side of your coat or clothes, showing that you keep those who died close to your heart.
It's also the side that military medals are worn on.
Others say men should wear it on the left and women on the right, like a badge or brooch.
The Queen wears hers on the left.
But the Royal British Legion says, "There is no right or wrong way to wear a poppy. It is a matter of personal choice whether an individual chooses to wear a poppy and also how they choose to wear it. The best way to wear a poppy is to wear it with pride."
Why do some people choose not to wear a poppy?
The Royal British Legion says wearing a poppy is a personal choice and it should be about your own thoughts and memories.
They say it is greatly appreciated by those it helps - those serving in our Armed Forces, former servicemen and women and their families.
However, not everyone chooses to wear a red poppy for many different reasons.
Some people feel the red poppy has become too political, and that some politicians have used the powerful feelings it creates, to justify war.
Others refuse to wear poppies because they feel there is now too much pressure put on people to wear them.