Thankyou for your answers
It depends completely on the hostel itself - they pretty much all have websites these days so shouldn't be too hard to look up their specific rules.
Usually, they all have rooms lined with bunkbeds. If you get a 'private' room then you'll have the room to yourself (usually you'll be able to lock the door) but if it's not specified, you could be lumped with anyone (and your group can be split up). Most hostels I've stayed in have not been en-suite so you're looking at sharing bathroom and toilets with other people. You can't leave things in your room without fear of it getting nicked but most places will have lockers.
Food and drink is usually (but not always) banned in the sleeping areas, some hostels allow alcohol in the shared 'living spaces' (kitchen/dining area/etc) but most do not allow alcohol at all.
A lot of hostels have curfews (usually around 10-11pm) so if you don't get back in time, you'll be locked out for the night.
I've stayed in some brilliant hostels and some awful ones. One of the worst hostels I've stayed in was when I was 17 with a group of friends in Brighton on the seafront, it had no heating (we were there at the end of January during a snowstorm) and you had to stick your feet in the shower if you wanted to sit on the loo and it was filthy.
Make sure you shop around read the reviews before you book!!
Unless you book a private room, you'll be in a shared dorm; no real privacy at all, as you'd expect from a shared room.
Generally you can come and go more or less whenever; you'd need to check whether the one you were looking at shut its doors at a certain time, or you're allowed food or drink (generally not)
Most have common rooms / areas where guests can gather.
They vary but tend to be unlike a hotel. There are more rules. You may have to sleep in a dormitory and alcohol is banned.
They do vary. One near me is all en-suite, and you can buy alcohol on the premises.
Look at the websites of the ones on the route you and your friend are planning.
Google them. No drink or noise allowed, they are nothing like a hotel.
They are clean and cheap but not a hotel why not look at the YHA website
Smelly as the Brits are unhygienic.