1. A computer database, comprising: a database store for receiving, storing, and allowing access to data concerning a plurality of topics, meta data created at a time of entry of said data, meta data comprising at least one annotation concerning said data, and meta data comprising access statistics concerning said data; a viewing tool for user access to said data in said database; an electronic tutor for maintaining a model of a user, and for finding useful data in said database to present to said user; an authoring tool for adding data into said database; and a data representation schema for organizing said database into explanations, topics, explanatory paths, and meta data representing information about and relationships between said data in said database; wherein an explanation comprises a piece of content that is linked with topics; wherein every explanation has links to at least one topic that it explains; wherein topics and explanations are linked by said paths; wherein said meta data comprises at least one annotation to said explanations and topics; wherein explanations also have links to topics that represent prerequisite data; wherein a topic comprises a cluster of concepts to be learned together by a user; wherein at least one topic comprises multiple subtopics; wherein at least one topic comprises a testable unit of data that contains no subtopics, and that represents a single entity such that a single question determines whether said user is familiar with said testable unit of data; wherein a path is a way of describing a sequence of explanations and queries, with branch points; wherein paths encode information about ways to learn a topic; wherein a path is connected to topics that it explains and topics that it depends on as prerequisites; wherein a path contains branch points that are based on answers to queries; wherein a path contains additional information indicating how a sequence is presented; wherein annotations are associated with at least one of: explanations, topics, paths, and other annotations; and wherein annotations do not modify an author's content, but add to it.