4. A system for searching for and presenting content items as an arrangement of one or more concept clusters to facilitate further search and navigation using at least one of a display-constrained display device and/or an input-constrained input device, the system comprising: at least one machine configured to perform steps of: accessing an electronically-readable storage medium containing a candidate set of content items; organizing at least some content items of the candidate set of content items into a hierarchical set of concept clusters, wherein at least two concept clusters in the hierarchical set of concept clusters each includes a respective set of content items, wherein the content items within each of the respective sets are related by one or more common themes or information types, and wherein at least one concept cluster in the hierarchical set of concept clusters has one or more cluster identifiers, and wherein at least one concept cluster in the hierarchical set of concept clusters is a parent cluster and comprises a child cluster, and receiving user input comprising more than one search term; identifying a concept cluster in the hierarchical set of concept clusters that has one or more cluster identifiers matching the user input, wherein the concept cluster in the hierarchical set of concept clusters having one or more cluster identifiers matching the user input is a parent cluster of a child cluster having a child cluster identifier; generating a flattened cluster based on a combination of the parent cluster in the hierarchical set of concept clusters having one or more cluster identifiers matching the user input and the child cluster of the parent cluster in the hierarchical set of concept clusters having one or more cluster identifiers matching the user input; and presenting the flattened cluster on the display device.
4. A system for searching for and presenting content items as an arrangement of one or more concept clusters to facilitate further search and navigation using at least one of a display-constrained display device and/or an input-constrained input device, the system comprising: at least one machine configured to perform steps of: accessing an electronically-readable storage medium containing a candidate set of content items; organizing at least some content items of the candidate set of content items into a hierarchical set of concept clusters, wherein at least two concept clusters in the hierarchical set of concept clusters each includes a respective set of content items, wherein the content items within each of the respective sets are related by one or more common themes or information types, and wherein at least one concept cluster in the hierarchical set of concept clusters has one or more cluster identifiers, and wherein at least one concept cluster in the hierarchical set of concept clusters is a parent cluster and comprises a child cluster, and receiving user input comprising more than one search term; identifying a concept cluster in the hierarchical set of concept clusters that has one or more cluster identifiers matching the user input, wherein the concept cluster in the hierarchical set of concept clusters having one or more cluster identifiers matching the user input is a parent cluster of a child cluster having a child cluster identifier; generating a flattened cluster based on a combination of the parent cluster in the hierarchical set of concept clusters having one or more cluster identifiers matching the user input and the child cluster of the parent cluster in the hierarchical set of concept clusters having one or more cluster identifiers matching the user input; and presenting the flattened cluster on the display device. 8. The system of claim 4 , wherein presenting the flattened cluster is based at least in part on a combination of the one or more cluster identifiers matching the user input and the child cluster identifier.