Worlds or universe is everything exists.
The first world is where the original Yuno Gasai originates from. Its events closely follow the timeline of the Second World, with minor differences up until the ending. As there was no 'fake' Yuno Gasai, there would only be two dead bodies. It is unknown whether the bodies were ever discovered. The winner of the Survival Games is Yuno after she and Yukiteru decided on a double suicide. Their method of death were drugs, however Yuno pretended to take them. After she is sure Yukiteru is dead, she spits the pills out and claims her victory as the god of the world.
Yuno had originally thought that after becoming god she could simply resurrect Yukiteru. However she can only resurrect the flesh but not the soul. As a solution she traveled to the past, thus creating the Second World. After arriving she slaughters that timeline's Yuno Gasai which meant there were three dead bodies. These are soon discovered by Yukiteru which greatly affects the future. It also creates the suspicion that Yuno is a fake, which in fact she is not, she's taken the Second World's Yuno Gasai's place. In the events of the anime, Aru Akise is constantly warning Yukiteru that Yuno is not to be trusted. After a series of investigating with Masumi Nishijimi, Yuno's secret is discovered.
This world was created after Muru Muru, Yuno Gasai, Yukiteru Amano, and Minene traveled to the past, which the setting is two years before the destruction. In the Third World no Survival Game takes place, making the world much happier. All of the deceased people from the Second World are alive in the Third World. Some examples of it being happier are: Marco and Ai are married, Ai pregnant. Second World Minene and Masumi are also married, in the anime it shows her children have inherited her powers. Yukiteru's parents never divorced, and in the anime they are seen together. John Bacchus and Kamado Ueshita are possibly dating. Yukiteru (the third world one), is dating Moe Wakaba. The third Yuno Gasai however, feels something is missing