For decades, the Democratic People's Republic of Korea - more commonly known as North Korea - has been one of the world's most secretive countries.
Its government doesn't like people from outside the country going there and finding out what's going on.
It shares a border with a country called South Korea - and the two countries have not got on with each other for a long time.
A North Korean leader had not stepped foot in South Korea for 65 years and the leaders of the countries had not met for 10 years.
But that changed in April 2018 when North Korean leader Kim Jong-un met with the president of South Korea Moon Jae-in at the border between the two countries. The meeting was a significant moment in modern history.
To better understand the relationship between the two countries, we need to look back over the last few decades at the history of this secretive country.
Korea used to be one country but was divided after the Second World War. It had been ruled by Japan but when Japan surrendered, forces from the Soviet Union occupied the north of the country while US forces occupied the south.
The Soviets supported a system of rule called communism, while the US was anti-communist. It meant the two sides could not agree on how to unite the country, so it was formally divided into North Korea and South Korea on 9 September 1948.
The political differences between the two rival states led to the outbreak of war in 1950, when North Korea invaded South Korea.
The war lasted three years but North Korea and South Korea have officially remained at war because a peace treaty was never signed.
Since 1948, North Korea has been ruled by three men from the same family.
Kim Il-sung was the country's first supreme leader, who was in charge until his death in 1994.
Control passed next to his son Kim Jong-il, who held power for 17 years.
In 2011, North Korea announced Kim Jong-il's son, Kim Jong-un, as its new leader.
Around one million people gathered in the capital city, Pyongyang, to hear the announcement.
North Korea is home to more than 25 million people, who live under a form of communist rule, which strictly controls all areas of daily life.
People have to ask permission to travel around and it's difficult for visitors to enter the country too.
All TVs and radios are tuned to state channels and people caught listening to foreign broadcasts face harsh punishments.
These controls mean that most North Koreans may have little or no idea of world events, or how their country is thought of by the outside world.
Most North Koreans are extremely poor with things like fridges, washing machines, and even bicycles, hard to come by.
Many people rely on aid agencies, such as the United Nations, to provide food because there is not enough to eat in the country.
However, North Koreans who demand more from their government, demand a change in leadership, or those that just try to escape, are brutally punished and sometimes killed.
An Amnesty International report estimated that hundreds of thousands of people have been put in prison and labour camps because they have disagreed with the government.
North Koreans seem to cheer and praise their leader at big public events but it's difficult to know the reasons for this because it's not possible to speak freely to people living there.
The North Korean government says it's because Kim Jong-un is very popular with his people.
From an early age, North Koreans are taught that their leaders are like all-powerful gods.
But others argue that people could be cheering Kim because they are worried that they would get in serious trouble if they didn't.
One of the main issues that has seen North Korea make headlines in the news is to do with nuclear missiles.
North Korea had been trying to make nuclear missiles - powerful bombs that can cause devastating damage - which one day might be able to hit targets in the US.
It wasn't believed that it was building any missiles that could travel to and strike the UK, which is around 8,400 kilometres away.
US President Donald Trump sent warships and submarines to South Korea, a country which America has supported for a long time, to try to get Kim Jong-un to stop his weapon-building plans.
In August 2017, North Korea fired a missile thought to be able to carry weapons, over northern Japan for the first time. It crashed into the sea.
Two weeks later, the country fired a second missile over Japan which flew higher and further than the first one. It also landed in the sea.
Japanese Prime Minister Shinzo Abe called the action a "serious" threat to his country.
In November 2017, North Korea said it had successfully tested a new type of missile that could reach the whole of the US.
In April 2018, the North Korea leader announced that his country would no longer carry out nuclear weapons tests.
He also said a nuclear testing site in the north of the country would be shut down.
President Trump tweeted that the announcement was "good news" for the world.
At the start of 2018, Mr Kim ended up in another war of words with President Trump.
Mr Kim made his New Year address saying he always has his nuclear launch button on his desk. Mr Trump responded by saying his button was bigger and the weapons he had were bigger and more powerful.
The relationship between the US and North Korea then seemed to improve as the two leaders announced that they planned to meet face to face.
It was a big deal because no serving US president has ever sat down for talks with a North Korean leader.
But Donald Trump pulled out of the talks. He said it was because of "tremendous anger" in a recent statement from North Korea and a series of "broken promises" from the North Korean capital Pyongyang.
However, Mr Trump said that he was very much looking forward to meeting him "some day".
At the same time, efforts were being made by North Korea to start friendlier talks with its neighbour, South Korea.
Mr Kim had said he was open to communicating and, in January 2018, a secure phone line was reopened allowing the two governments to talk to one another.
It had been almost two years since this phone line was disabled on the orders of Kim Jong-un, so this was an important development.
It came after the North Korean leader spoke of lowering tensions and of melting "frozen North-South relations".
After this, Mr Kim sent a North Korean team to the Winter Olympics in the South.
He said attending the games was "a good opportunity to show unity of the people" in North Korea.
South Korea and North Korea came together during the opening and closing ceremonies of the 2018 Winter Olympics in Pyeongchang, and the athletes were pictured side by side.
Following this, in April 2018, there was an incredibly historic meeting, when the leaders of North and South Korea met at the border between the two countries for the first time in 10 years.
Many people were hopeful that the talks between the two leaders were a sign of greater peace between the countries.
On 2 June 2018, it was announced that the summit between President Trump and Kim Jong-un in Singapore was back on.
Donald Trump and Kim Jong-un finally met on 12 June in Singapore, a small country in south-east Asia.
Their meeting started with a handshake and there were just two translators in the room with them.
After the two leaders spent around 40 minutes discussing some of the issues between the two countries, they invited all their advisors to join them for lunch.
The two countries then agreed a statement about what they wanted to do to improve their relationship.
Afterwards, as they both signed the joint statement in front of members of the media, President Trump said "we've had a really great time together".
Kim Jong-un added that "the world will see a historic change".
The two leaders met a second time in February 2019 and in June 2019, there was another historic moment when President Trump took a step across a demarcation line separating North and South Korea. It made him the first US president to visit the country.
Mr Kim even said to the president: "I never expected to meet you at this place."
The two men walked together into North Korean territory where they shook hands. After this, they walked back onto South Korean soil.
Mr Kim said their meeting was a symbol of their "excellent" friendship. Mr Trump said it was a "great day for the world".
The two leaders were only due to meet for a short time, but they ended up speaking with each other for about an hour and agreed to restart talks about denuclearisation, which had stalled.
In a news conference, Mr Trump confirmed that he had invited Mr Kim to the White House, but nothing has been formally arranged.
North Korea has continued to test missiles since then, and the two countries still sometimes use unfriendly language and insults to each other.
However in 2020, Mr Trump has said the two leaders have a "good relationship" and in March he sent a letter to Mr Kim offering US help in tackling coronavirus.