SEDA stands for the Staged Event Driven Architecture and is an architectural pattern for building massively scalable, distributed and concurrent systems.
There are some useful papers on the architecture. The basic idea is you pass event objects around on between separate threads/processes which have specific roles, making as much processing as possible asynchronous.
In ActiveSpace this is analogous with having a Space per thread/process group and adding an event Object (any Serializeable object) to the space and having some consumer removing it to process it.
By using the simple SEDA pattern you can easily build massively scalable grid type systems where you have a cluster of consumers pulling events out of a Space and producers drop events into a space. This provides good load balancing, horizontal scalability and fast failover if a consumer fails.