Asteroid 2014 DX110 will pass within the Moon's orbit at 1 p.m. PST (4 p.m. EST) today. It's not going to hit us, and we're not going to die in a planet-wide coordinated spectacular of death. At least, not today.
DX110 is about 30 meters (100 feet) across. Nothing is published about its composition, but you can make your best guesses and head over to the handy-dandy Impact Calculator (seriously, I love that thing) to find out what would happen if all the orbital dynamics calculations were totally wrong and we got smacked with it anyway (this isn't going to happen; we aren't going to die, blah blah blah don't spazz). From my own tinkering with "So, how bad could it get?" doomsday scenarios, even a dense rock that size would still break up in the atmosphere. As it broke up, it'd explode with the force of several kilotons of TNT, sending out an air pressure wave that would be loud and distressing to people nearby, but unnoticeable on the far side of the country. An impact that severe happens about every 200 years; it's an event of roughly the same magnitude as last year's Chelyabinsk meteor: spectacular, painful for anyone rushing to the window to stare up in awe, but not a planet-killing extinction event.
So, what's actually going to happen? DX110 will come within 350,000 kilometres (217,000 miles, or 0.9 the lunar distance) from Earth and then go about its merry way off into the rest of the solar system. Considering the crazy orbits of most asteroids, it'll be back eventually. Despite our limited capacities and resources for staring at black space for small dark rocks hurtling past at high speeds, we spot about 20 known asteroids coming between us and the moon every year. Just imagine how many we'd see if Spaceguard was properly funded and coordinated to make a serious search for near-Earth objects... Hey, maybe that's why the 2015 NASA budget has a line item for developing asteroid interception or deflection programs.
Image credit and DX110 stats via the JPL press release on AsteroidWatch. For more on the observing history of why we know DX110's orbit with this much confidence, check out the piece on Universe Today. If you want to geek out over orbital parameters, here's DX110's entry in the small-body database.