Lists are collections of people used for mailings, reports and many other things. Lists can be global, where they're available everywhere including all projects, or project-specific and only available in that project. You can also make temporary lists which last only until you exit VSys - these are great for "tagging" people for bulk operations.
Lists, both standard and temporary, are manually controlled and have people who are put on them individually or in bulk. This means that the lists change only when someone adds or removes people. Intellilists are different: they are essentially saved sets of rules which are automatically calculated. You don't add or remove someone from an Intellilist, instead VSys automatically determines who's on a list by whether or not that person meets the criteria of the list.