Sorry for such a basic question - to book hotel, does single means a single bed or a single person? Can a couple stay in a single room? I got a little confused that some hostels charges twice of the price for double as compare to single. I thought two people traveling together can generally save some money. What single really means? A twin-size bed? a full size bed or a queen size bed?
You can only book a single room for 1 person - no matter the size of the bed. For 2 people you either book a twin or double. A twin is 2 single beds made separately. A double is intended for 2 people to sleep together. In Europe it is often 2 single beds pushed together and made as one bed. This makes it easy for them to make up the room in either twin or double configuration.
I don't believe any hotel or B&B will allow you to book a single room for 2 people.
Of the smaller family run B&B and hotels, just accept that single means 1 person, Double-2, triple... What you get for a room beyond that can vary quite a bit. Modern chains (Formule 1 for example) do let rooms "American Style" where you pay for a room for up to 4 people. Generally hostels will charge per person, in other places the cost of a double is maybe only 50% more than a single.
Yes, they do mean different things. Twenty years ago a friend and I were in Aviemore, Scotland, and went to the tourist bureau to book a "double" room. When we got to the B&B, the stern Scottish woman took one look at us, and declared she was having none of that nonsense in HER place. She mistook us for 2 lesbians (not that there's anything wrong with it) and we had to do a lot of talking to convince her that we just hadn't understood that we should have booked a twin room. Perhaps we shouldn't have soothed her homophobia, but the place was really cute with a little heather garden out front.