15. An information retrieval system hosted and executing within a computer network comprising: an information retrieval component in communication with a search index and a collection of information; and a user interface module in communication with the information retrieval component, the user interface module comprising: a transceiver for receiving, during a defining session between a user and the information retrieval system, a user generated definition of an expression type wherein the expression type can be one of a query seeking information from the information retrieval component, a statement seeking to add new information to the information retrieval component, and a command to perform an action, the definition of the expression type specifying an expression format having a semantic structure, a response type, and a display format, and during an execution session between a user and the information retrieval system, a user generated expression having a semantic structure that matches a semantic structure that was received and stored during the defining session; a semantic evaluator for evaluating the semantic structure of the received expression by determining: (i) whether the semantic structure matches a command; (ii) whether the semantic structure represents an answer to a question; (iii) whether a query of the information retrieval component using the received expression exactly as phrased results in identification of stored information associated with the received expression; and (iv) whether the semantic structure is an implicit command to add data to the information retrieval component; a processor for processing the received expression according to the response type specified in the definition of the expression type for the received expression; a display component displaying a generated response according to the display format specified in the definition of the expression type for the received expression; and a storage device for storing the definition of an expression type.
18. The system of claim 15 , wherein the display format identifies a format for an element of a user interface displaying a response of the specified response type to an expression having the defined expression format.