What's planet and what does it have to do with 2012

I heard that 2012 there was a planet x that will come and collide with earth. Can someone explain this to me with more detail?

That's just an urban myth. You can read more about it here:
http://www.universetoday.com/14486/2012-…

Yes, it is the same Planet-X that killed us all on May 13, 2003.

The hoax (a hoax is a lie, told by people who know it is a lie) allowed some charlatans to make money back then, so they started it again.

In addition, it was made popular (more than in 2003) because of the internet and because of the movie "2012" (which is closer to the version of 2003).

There is no Planet-X coming to collide with Earth in 2012. It is a fake planet, invented for a hoax.

Sure... ready?
"Pfff.. heh.. um.. er... BWAHAHAHAHAHAHAHAAAAAAAa"

Now. That's the most involved answer you really need, but if you INSIST o knowing what the loony fringe is wanking about, it's called Nibiru, and it comes from Babylonian mythology. It DOES NOT EXIST, never did, never will.
http://www.google.com/search?q=nibiru&ie…

That's a hoax. And this question has been asked more than 40,000 times on this site. Please learn how to use the search function.

I hope you haven't spent any money to read this rubbish, as it is a scam being circulated by a fraudster

You could ask where you heard it from to explain it to you, why here? Here, we didn't hear nothing!

More bullshit by the government and NASA.