The world might be physically destroyed in one of three ways:
1) Sun expands and literally burns the very substance of the planet away. Might happen, might not happen - it depends on how weak the sun's gravity becomes as it gets larger. Earth might juuuuust avoid being incinerated completely if the sun's gravity becomes weak enough. But either way, Earth will eventually become uninhabitable.
2) Planetoid smacks right into us. There are roaming planetoids out there - chunks of rock almost as big as a planet that can careen into us and smash us to bits. A lot of geologists think that this very scenario happened a long time ago. If it's large enough, the Earth could be smashed into chunks, or, more likely, crushed into the planetoid, forming a new planet. Needless to say, everything on the Earth, probably even all the bacteria, would die.
3) Roving blackhole hits us. There are roving black holes out there. So far we don't think any are headed our way, but they are out there and they are numerous. If one passes by close enough, or hits our planet, we'd be dead before we'd know it.
Human civilization might expire in one of five ways:
1) Nuclear Warfare followed by a devastating nuclear winter. Less likely now given the reduced nuclear stock-piles. We did once have enough nuclear weapons to destroy all of humanity several times over. We no longer do, but we could always build more. Nuclear winters are also less likely due to the fact that better construction materials would reduce the fires caused by nuclear strikes, reducing the ash output. Most scientists think that we'd suffer a nuclear autumn instead of a nuclear winter.
2) Asteroid strike.
3) Plague - unlikely. We have pretty good quarantine measures these days. Any incredibly deadly disease that can spread fast also would have the Achilles heel of 1) Being noticeable and 2) Killing their host too quickly. Having said that, I can easily imagine a scenario in which a new disease could potentially kill up to 700 million people.
4) Gigantic volcanic eruption resulting in ash covering the sky, killing all surface life. Could happen. Could very well happen. And we have no way of stopping it or even predicting when it might occur.
5) Extreme Environmental Degradation, leading to the collapse of food stocks, leading to mass starvation, the general collapse of society and the destruction of mankind. Think Fallout, but without the radiation. Probably won't eliminate humanity entirely - there's so many of us after all. But survivors would be scattered and few and civilization would be reset to Zero.
