From a scientific point of view, a toxin is a very specific type of poison, one which is produced via living cells. For instance, a poison produced by a spider would be considered a toxin. A manmade poison, on the other hand, would not be considered a toxin.
In the modern world, most people think of toxins in a more general sense, simply as substances which are poisonous to humans and/or animals. They refer to poisons which are produced by living cells as bio toxins or natural toxins.