Ingredient is what you make it from. Component is what it contains.
The only significant ingredient for cider should be pure apple juice -- though perhaps small amounts of preservatives are allowed?
The main component is indeed water, which is the main component of apple juice (the other is the fruit sugar which makes the alcohol).
But why not buy decent cider that actually tastes of apples? You'd not have needed to ask then...
Strongbow, unlike proper cider seems to derive many of its fermentables from cheap adjuncts such as sugar, and rather than apple juice probably contains concentrate. I would imagine the main ingredient is water, with apple concentrate, sugar and possibly some other sugars (from corn, for instance) featuring below.