Until today, NASA was only able to directly view the Earth-facing side of the Sun. We could see solar storms as they happened, but not necessarily as they developed. Now, thanks to STEREO, we can see all sides simultaneously.
Our limited view was due to the fact that the Sun's roughly 27-day rotation hid the far side from our current crop of observational instruments, like the Solar Dynamics Observatory, or SDO. The SDO and its companion the Michelson Doppler Imager, while invaluable, can only produce a reconstruction of the activity on the far side of the Sun.
In a technologically-dependent world such as ours, this limited view was dangerous. Solar storms and coronal mass ejections, commonplace in our solar system, could easily build on the unmonitored far side of the Sun before launching toward Earth to knock out our satellites and on-world electronics. We could be, and have been, caught unaware.
To better prepare for these storms, NASA launched STEREO in 2006. Short for the Solar Terrestrial Relations Observatory, this pair of spacecraft follows Earth's orbit—one ahead and one behind—to offer unprecedented views of our life-giving Sun. As of today, they've finally reached a point along our orbit that gives a full 360-degree view of the Sun:
The current mission will continue for another eight years before STEREO A and B switch places on the far side of the Sun, and begin again. [NASA - Thanks, Alex]