Agent Factory - a agent that will make copies of itself. Usually these are mobile agents. Service - an API that allows for an agent to accomplish specific types of tasks. Usually there is security associated with access to specific classes of agent services. Hosting is simply a specific type of service.
Agents can offer services to local or remote agents as well.
Services I feel will be some type of interface which agents can query for. If a bridge node is removed the degree of connectivity will increase significantly, or the network will become unconnected. As opposed to a hub that must be referenced because of permanent limitations of dependent connections, it is a hub that may be referenced because of superior capabilities over its peers. For example, in a community of cars temporarily travelling together on the highway, a node may specialize as an analyst of real-time traffic data, and republish the data to interested parties. Minimum spanning tree there are a series of algorithm that will turn a graph into a tree.