1. A recommendations system for selecting items to recommend to a target user, the system comprising: a computer system comprising computer hardware, the computer system programmed to implement: a user interface that provides functionality for users of an electronic catalog of items to create and assign arbitrary textual tags to individual items represented in the electronic catalog after selecting said items, such that the textual tags categorize the items represented in the electronic catalog, the user interface comprising a tag entry field on item detail pages of the electronic catalog, the tag entry field enabling the users to create the tags via entry of text strings into the tag entry field; a recommendation engine comprising a tag-based recommender configured to: programmatically identify, from item preference data stored in computer storage, an electronic catalog item previously selected by a target user, without receiving a designation from the target user of the previously selected item, subsequent to identification of the electronic catalog item, programmatically identify one or more tags of the textual tags associated with the previously selected catalog item in a data repository by at least identifying a threshold number of most popular tags assigned to the selected item, without any input from the target user, the one or more identified tags having been assigned to the electronic catalog item via the user interface, and subsequent to said identification of the one or more tags, automatically search the electronic catalog using the one or more identified tags as one or more keywords to identify a corresponding set of related items; and a candidate selector component configured to: select at least a portion of the set of related items to provide as recommendations to the target user, and output the recommendations with associated textual reasons for recommending the items.
1. A recommendations system for selecting items to recommend to a target user, the system comprising: a computer system comprising computer hardware, the computer system programmed to implement: a user interface that provides functionality for users of an electronic catalog of items to create and assign arbitrary textual tags to individual items represented in the electronic catalog after selecting said items, such that the textual tags categorize the items represented in the electronic catalog, the user interface comprising a tag entry field on item detail pages of the electronic catalog, the tag entry field enabling the users to create the tags via entry of text strings into the tag entry field; a recommendation engine comprising a tag-based recommender configured to: programmatically identify, from item preference data stored in computer storage, an electronic catalog item previously selected by a target user, without receiving a designation from the target user of the previously selected item, subsequent to identification of the electronic catalog item, programmatically identify one or more tags of the textual tags associated with the previously selected catalog item in a data repository by at least identifying a threshold number of most popular tags assigned to the selected item, without any input from the target user, the one or more identified tags having been assigned to the electronic catalog item via the user interface, and subsequent to said identification of the one or more tags, automatically search the electronic catalog using the one or more identified tags as one or more keywords to identify a corresponding set of related items; and a candidate selector component configured to: select at least a portion of the set of related items to provide as recommendations to the target user, and output the recommendations with associated textual reasons for recommending the items. 17. The system of claim 1 , wherein the selected item comprises an item tagged by the target user.