1. A dialog system that uses a dialog move tree to manage a conversation between the dialog system and a user, comprising: a dialog manager to associate the conversation with a complex activity; and a plan engine to execute a plan script in connection with the complex activity, the plan script including a set of atomic dialog activities and logic to control a data and sequence flow of the atomic dialog activities, the set of atomic dialog activities being sub-activities of the complex activity, wherein: the dialog move tree forms a structured history of dialog moves performed by the dialog system and the user in the conversation; the dialog manager dynamically adds to the dialog move tree a node for each occurrence of an atomic dialog activity of the set of atomic dialog activities; and the complex activity is specified via a declarative activity specification language that connects the atomic dialog activities with a process.