Why is the concept of a list so important in SharePoint
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">
</BaseTypes>
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.

