Sharepoint 2003 alert function

I need to know if SharePoint alone has the capability to alert users when there are changes in documents, or when a task has been added to the portal when they are due or overdue, or a new version of a document is added to a document library and it needs review. Basically stated I need to know if and how SharePoint can alert owners of an action item to remind them that thier deadline is coming up and allow them to update their status on that action item or task

option to be activated.