It should be noted that the Schwarzschild radius only applies to spherical objects, and is a consequence of his description of the space-time geometry around any spherical object, not necessarly a black hole. Published in 1915, (soon after Einsteins general theory of relativity), it makes the remarkable prediction, if you take any spherical mass and compress it, when its size hits the Schwarzschild radius the gravity will be so strong; the space-time so bent, not even light can escape. The 'surface' this radius describes is more commonly called the event horizion, as you cannot see past it; the light of your torch would disappear, no reflections would return to carry information to your eyes.

This radius can be as small or as big as you like, you simply have to have enough mass within it; and so you can have black holes smaller than the size of an atomic nucleus, through to the size of the solar system, or even bigger... Interestingly, if you churn through the equations, looking at how the density of the object changes with radius, you find the density goes down as the radius goes up. So by the time you get to a supermassive black hole the size of a solar system, its density is comparable to water...

In pursuit of generality; trying to find out what happens to massive irregulary shaped objects, Kip Thorne formulated the hoop conjecture. In this there is a critical circumference of the mass in question which determines whether or not collapse to a black hole is inevitable. if the object can be passed through the a hoop with this circumference, it will collapse to a black hole.