A raw List can hold any type, whereas a List<?> holds an unknown, but specific type. That is, List is a heterogeneous container, while List<?> is a homogeneous container. However, I had observed that the compiler tends to accept either without warning, so I wrote the following example to explore this:

import java.util.*;

public class UnboundedWildcards1 {
  static List list1;
  static List<?> list2;
  static List<? extends Object> list3;
  static void assign1(List list) { 
    list1 = list;
    list2 = list;
    // list3 = list; // Warning: unchecked conversion
    // Found: List, Required: List<? extends Object>
  }
  static void assign2(List<?> list) {
    list1 = list;
    list2 = list;
    list3 = list;
  }
  static void assign3(List<? extends Object> list) {
    list1 = list;
    list2 = list;
    list3 = list;
  }
  public static void main(String[] args) {
    assign1(new ArrayList());
    assign2(new ArrayList());
    // assign3(new ArrayList()); // Warning:
    // Unchecked conversion. Found: ArrayList
    // Required: List<? extends Object>
    assign1(new ArrayList<String>());
    assign2(new ArrayList<String>());
    assign3(new ArrayList<String>());
    // Both forms are acceptable as List<?>:
    List<?> wildList = new ArrayList();
    wildList = new ArrayList<String>();
    assign1(wildList);
    assign2(wildList);
    assign3(wildList);
  }
}

So even though List and List<?> are technically different types, the compiler treats them interchangeably.