Java immediately instantiated class [duplicate]

Question

Can someone please explain to me how this code works? Specifically, the new Comparator part? It looks like a class definition that's being immediately instantiated. Thank you.

// Make a List of all anagram groups above size threshold.
List<List<String>> winners = new ArrayList<List<String>>();
for (List<String> l : m.values())
    if (l.size() >= minGroupSize)
        winners.add(l);

// Sort anagram groups according to size
Collections.sort(winners, new Comparator<List<String>>() {
    public int compare(List<String> o1, List<String> o2) {
        return o2.size() - o1.size();
    }});

// Print anagram groups.
for (List<String> l : winners)
    System.out.println(l.size() + ": " + l);