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Can anyone explain IEnumerable and IEnumerator to me?
What are the differences between IEnumerator and IEnumerable?
IEnumerable is an interface that defines one method GetEnumerator which returns an IEnumerator interface, 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();
An IEnumerator is a thing that can enumerate: it has the Current property and the MoveNext and Reset methods (which in .NET code you probably won't call explicitly, though you could).
An IEnumerable is a thing that can be enumerated...which simply means that it has a GetEnumerator method that returns an IEnumerator.
Which do you use? 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. But you'd still need to return that IEnumerator in an IEnumerable class.
For a look at what an enumerator (implementing IEnumerator<T>) looks like, see any Enumerator<T> class, such as the ones contained in List<T>, Queue<T>, or Stack<T>. For a look at a class implementing IEnumerable, see any standard collection class.