We have to define what the world ending would mean... Are we talking about the planet being completely destroyed somehow, or the local solar system somehow being destroyed? Or are we talking about the end of all life on earth? And if the latter is the issue, are we considering a temporary complete loss of life or does the loss of life on earth have to be permanent?
I've always heard that the sun is in the middle of its life cycle and will eventually burn out and not be able to support life on earth anymore. This just goes toshow that there are so many theories out there, it's hard to determine which one is even close to being true.
We do know that eventually the Earth will be swallowed by the Sun. As the sun converts helium to hydrogen, it will expand. Billions of years from now, the area that the Earth occupies in space will be occupied by the sun as it becomes a red giant. Of course, the Earth could move out of the way. It is also possible that the Earth will become uninhabitable before the sun becomes a red giant simply due to the changes in our orbit and proximity to the sun. Science is unsure exactly what will happen so far in the future. At some point after the sun becomes a red giant, it will go nova and probably wipe out this part of the solar system. If this event were to occur today, the human race would also cease to exist. Since the event will actually occur so far into the future, it is impossible to know what type of technology the human race will possess. We might be living on other planets or on space craft. Who knows what will happen to our inventive and adaptive species.
The Earth has survived many cataclysmic events in its long and storied history, from meteor impacts to intense volcanic activity, ice ages, etc. That same history has also shown that species, humans included, have a much more tenuous existence on the planet. Humans seem to have a tendency towards self destruction, and the planet cannot sustain us forever.
We know that our planet and anything still living upon it will essentially be destroyed in billions of years when our sun goes nova. At that point, there will no longer be life on Earth. So in a sense, all life on Earth will end in the long run.
I agree with the previous post, generally when people talk about the end of the world, what they are referring to is the end of the human race, not necessarily the end of all life as we know it. As the history of the planet has shown, enormous cataclysmic changes happen from time to time, and creatures survive or perish based on their ability to adapt to them. On the other hand, the period in which mankind has inhabited the planet is the blink of an eye compared to dinosaurs, for example.
The answer to either is one which no one can be absolutely sure. Historically, our planet has seen many animals become extinct or endangered. Our planet, on the other hand, has been able to survive pollution, abuse, drastic temperature changes, and billions of people existing upon it for around 4.54 billion years. That said, one can only assume that the world will be able to exist for a very, very long time.
The ability to survive for the human race, on the other hand, is far to determined by a multitude of other factors. Global war, global natural disasters, weapons of mass destruction, and other destructive "things" can end life on Earth as we know it. Unfortunately, one cannot foresee any of the above. One can only, instead, assume that there are things which could end the human race.