A colloquial description of a Swarm is that it is a way to represent a community of agents.
Communities can be embedded in other communities, or they can be independent.
A Swarm serves two purposes:
A container
Agents or components that are a part of a Swarm typically will use that Swarm as their Zone. Thus, when a Swarm goes away all the components in that Swarm go away two.
A common temporal space
At a given instant in time, many things may be scheduled to happen in a simulation. A Swarm keeps track of the ordering of events, translating the agents subjective experience of time into a single objective sequence of events.