Well the NUS is the national union of students kind of like a trade unions they organise protests and the like and look after the needs and rights of students you can join at 16.
At university the students union really comes into it’s own most universities have a union bar where you can buy drinks and food (I often got lunch there) also a lot of social events and things like student showcases are done there (some are quite good) there is a lot done for freshers too by the local SU (that’s the union of the university your at .) They will let you know about clubs and societies if you ask ( there will be loads at the freshers fair but they can give you contact details of the captain and stuff if you want to give something a go later in the year. They are there for students if you are having trouble finding a place to live they have an accommodation office and can help you there. If you are called to an academic miss conduct hearing you can ask for a rep to make sure everything goes smoothly they will put on events as I said before if a club is going on a trip like to a snow dome (guess who’s a skier) they have a mini bus they can use. You can go to them if you feel you have been unfairly treated and they will “go into bat for you” they can also advise you on legal matters I think they can also help with financial hardship funds too it’s all run by students and is vital for smooth running of the university from the student side don’t worry about it too much it will all become clear by the time you get to university.