We're hearing a lot about releases of radiation these days. The nuclear power station at Fukushima Daiichi is releasing radiation. Radiation has been found in Japanese milk, spinach, seawater, etc. Nuclear technicians wear those white or pastel-colored papery body-suits to protect themselves from radiation. Living near a coal-fired power station exposes you to radiation. And so on and so on.
The problem is, when the word "radiation" is used like this (as it is, colloquially and in news reporting) it interferes with understanding of what is going on inside the power station. So radiation is being released - but that doesn't tell us the danger, that doesn't tell us what's causing the problem, that doesn't tell us how far away people should be before they feel safe.
A quick run-down of what goes on inside a nuclear reactor (and I am not a nuclear engineer, so feel free to correct me):
Fuel rods, which are pellets of enriched uranium inside a hollow metal tube, are packed together and immersed in a working fluid like water, liquid sodium, helium gas, or whatever. Somehow, neutrons come in - I am not sure if a neutron source has to be introduced, to make a reactor go critical; but when a neutron hits a nucleus of uranium, it splits apart and releases heat and more neutrons. These neutrons fly out and have a good chance of hitting other atoms, which will also release heat and split, and so on. This forms a nuclear chain reaction. The heat comes from the difference in mass between one atom of uranium and the fission products of one atom of uranium - the fission products have less mass, around one-half the mass of a proton, so this missing mass gets converted into energy (a bit less than 1.5 x 10^-10 joules, if I did the math right). Some of this energy is heat, which gets transferred to the working fluid, and some is in the form of high-energy photons (gamma radiation).
So, again: the products of fission are heat, neutrons moving at various speeds, and gamma ray photons traveling at the speed of light. If you remove the heat, what you have (neutrons and gamma rays) is radiation.
There are two other things that get called "radiation", though. First, the decay products of uranium (like weird isotopes of strontium) are often themselves intensely radioactive. Second, when neutrons hit atoms more stable than uranium, they get captured and transmute whatever they've hit into a different element. This new atom is often unstable, and so it decays into a more stable state by emitting some form of radiation. In other words, normally inert things that are inside a working nuclear reactor often become radioactive themselves. If there's a fire in the reactor, or among spent fuel rods, or if coolant sprays into the air through leaking pipes, radioactive material will be released into the environment.
The confusion in news reports comes about because they use "radiation" to refer to both the radiation released by fission itself, and to radioactive material that comes from fission. One is released instantly, and if the reactor is shut down - for example, by having the neutron-absorbing control rods shoved in all the way - it stops. The other is lasting, and if reactor components or radioactive coolant are released into the environment, it becomes an issue of persistent pollution.
If observers near a damaged (and allegedly shut down) reactor building detect gamma radiation and lots of neutrons, they know that there is a critical mass somewhere in the building, and it is undergoing fission. But if they detect radioactive particles in the air or water or on the ground, they know there is either a coolant leak, a fire in the reactor building, or both.
With gamma rays and neutrons, the intensity of a person's exposure will decrease very fast as he moves away from the reactor. It follows the inverse square law. Also, gamma rays and neutrons can be absorbed, whether entirely or only somewhat, by dense material. So if you're running away from a criticality accident, where a critical mass has formed and gotten out of control, you could climb over a hill and be safe from it (distance plus shielding). But with radioactive particulates or gases, to avoid exposure, you have to stay upwind from the burning or leaking reactor. Otherwise, chances are you'll inhale them or they'll rain down on you. This is why there's still a radiation hazard, after a nuclear bomb goes off. Sure it releases a burst of neutrons and gamma rays at the moment of detonation, but once that's gone it starts raining transmuted (and therefore radioactive) debris. It's the same basic phenomenon.
In the event of a reactor accident, anywhere... if nothing else, the public deserves to know which way to run.