Nanotechnology is of two types: highly miniaturized mechanical robots or modified cellular structures. The ultimate goal with the former is the creation of an assembler nanomachine that can build copies of itself. In the second case the focus is on molecular manufacturing. There is concern that either variety could get out of control and proceed to modify the environment at an exponentially increasing rate, possibly converting it into what has been described as a "grey goo".
The problem of someone making a self-replicating machine that could run wild has been overemphasized. It is a technical possibility which needs to be avoided. But it is a relatively minor problem because there is relatively little incentive for someone to do it. In addition it is difficult to do and there are many more obvious ways to abuse the technology.