A Lookup data page returns a record based on a primary key.
A List data page returns a collection of records based on the data source specified.
So, one return a single page whereas the other returns a page list. A List data page is typically used as a data source for a list-based control, like a Dropdown or Radiobuttons. Once a specific record from the control is chosen, a Lookup can be performed to retrieve the details of that particular record.
If you are familiar with Obj-Open and Obj-Browse methods, then Lookup data page is like Obj-Open method (where we provide the keys of the class to open any particular record) and List data page is like Obj-Browse method (that returns a collection of records).