Reactive Robotics

In order to increase the responsiveness of the early robots the plan element of the sense, plan, act sequence was dropped in what became known as 'reactive robotics'. The paper Intelligence without representation (1991) by Rodney A. Brooks has an introduction which explains the ideas behind the concept and then goes on to explain one of the most well known examples of the reactive architectures - the subsumption architecture. 

Subsumption works by defining the robot's behaviours and organising them on to the 'subsumption stack'. Each layer has access to the sensor input and can specify the action to be taken. The higher layers are more abstract and work to achieve the robot's overall goal whereas the lower layers are simpler actions which need to respond immediately to environmental stimulus. The details of the architecture are well worth looking into. Reactive robots are still in use especially in swarm robotics.     

Check out YouTube for some examples, I found this one illustrating an avoid behaviour: