Last night, an asteroid passed about 270,000 miles away from Earth, just further away than the moon. Some news organizations called this a "huge" asteroid. This is an overstatement. The asteroid is about half a mile across, which is larger than most asteroids, but much smaller than the largest asteroids (nearly 500 miles across, or 1000 times larger in diameter).
1/2 mile diameter is big enough that we need to keep an eye on this object, 2004 XP14. With computers, we are certain that 2004 XP14 will not hit the Earth in the next 200 years. It is hard to predict with certainty for much larger periods of time until we know more about the asteroid. Still, in the far distant future, 2004 XP14 could very well hit the Earth. An asteroid of this size would cause tremendous damage on a local level, and could destroy a city, but the effects would not reach further. If the asteroid hit the ocean (quite likely, since 3/4 of the Earth is water!), then a tsunami might be generated, but the effects would depend on where the asteroid would hit.
So, no need to worry for now -- Earth is still safe for the foreseeable future.