You've probably heard that staring at the sun is bad for your eyes. The reason you've heard this is that people who stare at the sun can go blind. Here's why. When you were a kid, you may have performed the trick where you lit paper on fire using the sun and a magnifying glass. The light of the sun is so strong that, if you concentrate it with a lens, you can actually start a fire.
In your eye, you have a lens. If you stare at the sun, this lens concentrates a spot of sunlight on your retina, and it burns it too. The light is so intense that it kills cells on your retina.
So much hype surrounds staring at a solar eclipse because, on a day-to-day basis, most folks know better than to stare up at the sun. The problem with a solar eclipse is that it's an extremely rare event. In fact, all of us here in the United States are going to have to wait until the year 2017 for the next solar eclipse (it's path of totality will run from the west coast of Oregon to the east coast of South Carolina). Because these eclipses hardly ever happen, everyone wants to see them when they do occur. People are tempted to do what they know they shouldn't, thinking that a few seconds of looking at the eclipse won't do any harm. Usually, they believe they're right, because no initial pain is associated with a retinal burn. Generally, it takes several hours for the symptoms to manifest and by then, the damage has already been done.