4. A method comprising: generating, at a first user device, a first hierarchical taxonomy comprising a first plurality of topics, the hierarchy being represented by a first tree having a first plurality of nodes, each node corresponding to a respective one of the first plurality of topics and indexed by a respective identifier; receiving a selection, at said first user device, from a first user, of a respective one of the first plurality of nodes of the first tree; generating, at the first user device, a first message, the first message comprising a subtree, the subtree comprising the selected one of the first plurality of nodes and all nodes within the first plurality of nodes located hierarchically below the selected one of the first plurality of nodes in the first tree, wherein the nodes in the subtree are hierarchically arranged as in the first tree; and sending, over a network, the first message to a second user for inserting, at the second user device, the subtree into a second tree, wherein the second tree represents a second hierarchical taxonomy comprising a second plurality of topics, the second tree comprising a second plurality of nodes, each node corresponding to a respective one of the second plurality of topics and indexed by a respective identifier.
4. A method comprising: generating, at a first user device, a first hierarchical taxonomy comprising a first plurality of topics, the hierarchy being represented by a first tree having a first plurality of nodes, each node corresponding to a respective one of the first plurality of topics and indexed by a respective identifier; receiving a selection, at said first user device, from a first user, of a respective one of the first plurality of nodes of the first tree; generating, at the first user device, a first message, the first message comprising a subtree, the subtree comprising the selected one of the first plurality of nodes and all nodes within the first plurality of nodes located hierarchically below the selected one of the first plurality of nodes in the first tree, wherein the nodes in the subtree are hierarchically arranged as in the first tree; and sending, over a network, the first message to a second user for inserting, at the second user device, the subtree into a second tree, wherein the second tree represents a second hierarchical taxonomy comprising a second plurality of topics, the second tree comprising a second plurality of nodes, each node corresponding to a respective one of the second plurality of topics and indexed by a respective identifier. 5. The method of claim 4 further comprising: updating a node in the first tree at the first user device, corresponding to the selected node in the updated tree at the second user device, wherein the node in the first tree at the first user device is updated to reflect the modifications to the selected node within the updated subtree.