Retrieval involves the processes by which stored information can be recalled, from handwritten indexes and card catalogs to digital tagging and search engines.
Classification involves the logical arrangement of information for purposes of finding it quickly when it is needed. This is trickier than it sounds, as different words can be used to express the same or similar ideas, and people organize ideas according to their unique understanding and vocabulary. The arrangement structure itself is referred to as a taxonomy or an ontology.