The model swarm alone defines the simulated world. But an experiment does not just consist of the objects being experimented upon, it also includes the experimental apparatus used for observation and measurements. In Swarm computer simulations, those observation objects are placed in an observer swarm.
The most important object in an observer swarm is the model swarm that is being studied. The model swarm is one component of the observer, kind of like a little world in a petri dish on the lab bench. Other observer objects can then input data into the model swarm (setting simulation parameters, for instance) and read data out of the model swarm (collecting statistics of the behavior of agents).
Just as in setting up a model swarm, an observer swarm has a collection of objects (the instrumentation), a schedule of activity, and a set of inputs and outputs. The activity of the observer schedule is to drive data collection - read this number out of the model, draw it on a graph. The inputs to the observer swarm are configurations of the observer tools: what sorts of graphs to generate, for instance. The outputs are the observations.
When running in graphics mode, the observer swarm objects are largely used to mediate user interface. For instance, in Heatbugs the HeatbugObserverSwarm creates Raster widgets, EZGraphs, and Probes. All of these objects are connected into the HeatbugModel swarm to read data, and to graphical interface objects so the human sitting in front of the computer can observe the world.
Interactive, graphical experimentation with models is useful for coming up with intuitions. But for serious experimentation it is necessary to collect statistics, which means doing many runs and storing data for analysis. As an alternative to a graphical observer swarm, you can also create batch swarms, observer swarms that are intended to be run without any interaction at all. Instead of putting up fancy graphics, batch swarms read data from files to control the model and write the data out to other files for analysis. The key here is that the model swarm used in the batch swarm is the exact same model as that used in a graphical observer swarm: the only difference is what tools the observer (batch or graphical) connects to the model.