Examples:

public IEnumerable<Class> ReturnClass()
{
    var list = ...;

    return list.Select(item => item);
}

or

public IEnumerable<Class> ReturnClass()
{
    var list = ...;

    foreach (var item in list)
    {
        yield return item;
    }
}

Are there differences in the generated code? Performance? Etc

I only found one question on stackoverflow about this subject, but it raised more questions than it answered questions