How best to describe a solar sail

CBC's Bob McDonald tells us here:

A solar sail is basically a potato chip bag - one the size of a small town - that is unfurled in space. The thin, shiny, lightweight Mylar plastic will reflect sunlight off its enormous surface and experience a small force.

Bob McDonald is a bit like Neil DeGrasse Tyson in his excitement for and ability to explain scientific and astronomical concepts, and every once in a while something like that stands out as a perfect way to explain a concept that otherwise might be a bit difficult to explain.

The rest of the blog post is worth reading as well as it explains an upcoming solar sail mission scheduled for next year, and surmises a bit on why solar sails have been so neglected compared to other types of propulsion. A Russian mission attempted to launch one in 2005 (a mission I was quite looking forward to at the time), but the launch failed and thus we have still failed to see a solar sail in action.

Wikipedia also has an interesting image here detailing the trajectory a solar sail would take to achieve maximum velocity - first it would begin on a trajectory taking it away from the Sun, but at too low a velocity to break free. Then the Sun would begin to pull it in and it would keep its sail turned towards the Sun in order to avoid falling directly into the Sun. The Sun does not manage to entirely pull it in however and the solar sail is able to maintain a close but significant distance from the Sun, which means that instead of being pulled in it is able to use the Sun's gravity as it passes by, and as it passes by it adjusts the angle of the sail in order to benefit from the sunlight at the same time, and ends up using the gravity and sunlight in order to achieve a velocity of 30 AU per year.

As for why flybys of the Sun are not used by other craft, see here.