A LiveSpace Entity is a DataBean that can be shared across the Livespace Bus. It is hosted in an EntityContainer, which can either be an EntityServer or an EntityClient, depending on whether the Entity is being hosted or referred to by a client. The Livespace Bus is essentially a set of entity clients and servers exchanging entity updates.
As a DataBean, an Entity is essentially a collection of name/value pairs, where the value can be any type including other DataBean's. This allows heirachies of DataBean's to be created. DataBean's provide clients with the abilty to listen for changes to their state i.e. when properties change. Unlike many similar systems, changes in the heirachy are also easily monitored.
See the Livespace Development Documentation page for more information on Livespace entities. The Entity JavaDoc page has more information on the Entity Java API.