Description lists (dl) are block-level containers, much like ordered lists (ol) and unordered lists (ul), and are used for presenting and defining terms.
Description lists (dl) contain sets of description terms (dt) along with corresponding description definitions (dd). Each description term has at least one description definition (dd) associated with it.
According to HTML5 Doctor, definition lists are often used for glossaries and for metadata. If the definition list is being used as a glossary, then a definition term needs to be specified as being a definition (dfn).