Apple cider vinegar is made by fermenting apple cider. It is sweeter, fruitier and milder than wine vinegar, which makes it suitable for many uses.
I use it in quick marinades for raw vegetables, sweet and sour sauces, to deglaze red cabbage (the acidity sets its colour) and in vinaigrettes. Choose an unpasteurised, slightly cloudy apple cider vinegar: it is a sign that it hasn't been filtered and keeps all its flavour.