Real cider is a 100% full juice product, made from apple juice and nothing else. (There may be a tiny amount of sulphite added after pressing to kill bugs, but that's driven away by the gas during fermentation) You will typically find real cider from small and medium size producers, many of whom grow their own apples and will be happy to show you their press and other facilities. Real ciders each have a unique flavour and bouquet depending on the producer, apples, and season, in this way they are like fine wines.
Most mass-market cider (not all!) is so-called industrial cider, made from imported concentrated apple juice. They add dextrose from hydrolysed corn starch to the concentrated juice, then ferment it with a so-called "turbo" yeast to about 28% alcohol by volume, then back-dilute it with water to the 4.5% or whatever they sell it at. They will then pasteurise it to kill the yeast, add colourings to get a consistent finish, sweeten it with an artificial sweetener to give it a sickly sweet flavour, and carbonate it with industrial CO2. Because all cider is bright orange, sweet and fizzy, right?
The resulting product can have <30% apple juice content, and have more in common with an apple-flavoured alcopop than a cider. You're being sold a product as "cider" that isn't really a cider at all.
How to spot the good stuff? They'll all tell you a story about oo-arr farms and orchards and heritage, even if they're really a multinational with a huge industrial plant somewhere. Read the small print on the bottle, if it's a high juice cider they should put it on the label as it's a selling point. Look at price, size of producer, then look and taste of the drink. Most real ciders are pale in colour, flat, and dry. Bright luminous fizzy sweet drinks like Magners are everything a real cider shouldn't be. Taste, there should be a significant tannic bite rather than a mild taste of apples. (exception: eastern counties ciders that don't use tannic apples. But you won't see many of them!) Find a single varietal, maybe Thatchers Katy or Westons Dabinet, find a cider like Hogan's, Henney's, Orghard Pig, Westons Reserve, or Gwynt y Ddraig, then compare its taste with Magners or Strongbow. You'll see the difference.
Now, "scrumpy". From west country dialect: "scrump", "withered apple". Applied as a marketing term, means nothing. Beware if farm gate cider is sold as such, it might be "Undrinkable crap they sell to tourists".
So there you are. Now you know something about cider. Enjoy exploring the different ciders you can buy, some of them are very good indeed.
(Slides, a selection from a much longer talk I've given in the past to my hackspace. Maybe I'll give the full version again some time.)