Yes, you share with others unless there are enough of you to get a private room.
Generally they are used by walkers, climbers, cyclists, etc, so an OK crowd. We're not talking homeless hostels here. Lots of families too.
Typical format is a big old house (often old manor houses etc) with doom rooms upstairs and a self catering kitchen and lounge area / common room downstairs.
Dooms tend to be 4 to 6 bunk beds, with clean sheets and pillow cases supplied when you check in.
Shower rooms and loos are also shared, but there's more than one, I've never had to wait for one yet.
I've used them in Lake District, North Wales, South Wales and Cornwall this year and all have been just fine, although each is a little different.
Like I say generally not posh, but friendly, clean and safe.
As far as leaving the bike, usual common sense applies, in depends on the location.
Out in the sticks in North Wales should be OK, St Pancras or Oxford St in London, or central ones in Manchester or Livepool, not so much. Most have parking, some don't.
Most serve bottled beer (including the local real ale), some serve evening means and breakfast for around a fiver extra.
For details see:
http://www.yha.org.ukIt's a little different in format than a hotel, but works well for me. It's great to be able to just pick a spot where it aint raining and go, without shelling out £60+ on a hotel.
Hope this helps, Andy
PS) I should add that I'm talking specifically about hostels run by the YHA (which is a charity). Other random hostels and in particular international ones can get be dangerous.