Lists are lists of keywords or tags. A Lists' primary usage is to serve for tagging content.
A list is your controlled vocabulary, your taxonomy which is used for tagging with multi-dimensional information. That means a list can only have a list of tags with a name, or a list of tags that have further information like synonyms, translations, broader terms, narrower terms etc.
A list in Picturepark can also be linked to other lists which then enriches the tag as all information from linked lists travels with the tag e.g. a person "John Doe" tagged with department "Marketing" has all additional information about the marketing department like goals, manager, other team members).