What is the difference between Enumerable and IEnumerator?

IEnumerable is an interface that defines one method GetEnumerator which returns an IEnumeratorinterface, this in turn allows readonly access to a collection. A collection that implements IEnumerable can be used with a foreach statement.

Definition

IEnumerable

public IEnumerator GetEnumerator();

IEnumerator

public object Current;
public void Reset();
public bool MoveNext();

The only reason to use IEnumerator is if you have something that has a nonstandard way of enumerating (that is, of returning its various elements one-by-one), and you need to define how that works. You'd create a new class implementing IEnumerator.