List is a powerful concept in SharePoint. It forms the basic storage mechanism. Everything in SharePoint be it simple item stores like tasks, customlists or more complicated ones like surveys, Calendar entries, project tasks, blogs, wikis, etc are all internally stored as list entries. Infact even Document Library leverages the list infrastructure.
If you open the Onet.xml file in the Global Template directory ( <Installation Drive >\Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\GLOBAL\XML\ONET.XML, The following nodes will be seen under Base Types
<BaseType Title="Generic List" Image="/_layouts/images/itgen.gif" Type="0">
<BaseType Title="Document Library" Image="/_layouts/images/itdl.gif" Type="1">
<BaseType Title="Discussion Forum" Image="/_layouts/images/itdisc.gif" Type="3">
<BaseType Title="Vote or Surevy" Image="/_layouts/images/itsurvey.gif" Type="4">
<BaseType Title="Issues List" Image="/_layouts/images/itgen.gif" Type="5">
Any list that will be used in SharePoint will be from one of these base types. What is seen as Type here is the ID of the list. As I understand this classification is mainly because the basc purpose and the storage mechanism will be slightly different with each of these.