One of the largest known solar flares erupted from the sun on Tuesday, producing a storm of superheated gas that's predicted to hit Earth by mid-day Wednesday.

Solar flares are categorized on in three categories, with X being the strongest. Within each category, there's a number (from 1 to 20) that describes the intensity.

If I understand this correctly, the numeric portion is measured like the Richter scale that measures earthquake activity. For earthquakes, a magnitude 4.0 is 10 times more powerful than a magnitude 3.0 quake. And a 5.0 is 10 times more powerful than a 4.0. With a solar flare, the X17 that just occurred is way up there on the top of the scale, especially given that X20 is as high as we measure and the only outbursts that were more powerful than the X17 (in recorded history) were two X20's. Additionally, the recent solar burst happened to be directed straight at Earth, which is something that has not happened in the past.

Consequences? Probably nothing devastating, but communications systems might go down for a while and various electronic systems might freak out. Obviously those kinds of things could indirectly cause devastating results (for instance, if hospital equipment were to malfunction), but those kinds of cases will likely be isolated and certainly nothing really crazy like people melting if they look at the sun during the solar storm.

I found this cool story about the solar storm of 1859, apparently the oldest solar storm we have on record.

General info from this article, and more specifics from spaceweather.com's main site or their page about solar radiation storms.