I have to agree with esmithiii, but if you are breaking in a new tank then a bit more detail would be helpful. .25 ppm is generally not going to hurt anything and is normally the higest level briefly reached when adding a new fish or making other changes on an ESTABLISHED tank. At .5ppm you will start seeing distress among the shrimp and other ammonia sensitive critters. At 1ppm you will start seeing distress among the less sensitive critters. At 2ppm all but the hardiest fish will be dying or dead.
The toxicity of ammonia is highly dependant on the pH of the tank water... the higher the pH, the more toxic ammonia you have (versus ammonium). But, yes, 0 is the target value and a healthy cycled tank should maintain 0ppm.
