Surveillance is a mode of power based on the detailed and totalized observation of behavior. It is, according to Michel Foucault, the prevailing form of power developed and deployed in modernity. It is grounded in knowledge and aimed primarily at control rather than repression.
Behavioral observation leads to generalizing schemes distinguishing "normal" from "abnormal" behavior, and such schemes can be infinitely detailed - features of behavior, big and small, are usually aggregated into equally generalizing "profiles" of people, and in building such profiles any and all details of behavior can be included. Thus, when approaching airport security checks, "suspicious behavior" can be detected on seemingly trivial grounds such as sweating, nervous facial expressions, gazing at the security officers, or even "looking too normal" - all of which can be perfectly innocent and have multiple causes. None of this behavior can be said to be a violation of laws - it is not punishable as such. But it can be read as a violation of broadly sketched "social norms" for behavior, indicative of the potential to transgress in more serious ways.
The awareness of surveillance - for instance, awareness of the presence of surveillance cameras - can lead to behavioral adjustment: we tend to try to look as "normal" as we can, and display what is called "insouciance", a pretended lack of interest in the fact of being observed. And we are likely to lower the speed of our car when we know that speed cameras are in operation. Surveillance is, in that sense, a technique of control: it is aimed at "rectifying" behavior before it becomes a violation of norms of laws.
Surveillance is the principle behind digital data gathering, and it leads to algorithmically configured "profiles" of users guiding (and, ideally, also changing) users' behavior.