Hostels are an accommodation generally for youth people and probably the cheapest ones. These two facts are tightly connected. Most young people love travelling but they usually have to be very economizing in means because they earn not very much and can't afford themselves luxurious vacation. Well, sometimes they can't afford themselves even the medium range vacations.
In hostels there even used to be some age limits though nowadays their doors are mostly open to the people of all ages.
In hostels you share one dormitory with other people. Dormitory is one common bedroom where beds are situated with the minimum space between each two of them. Sometimes there are even bank-beds. In your personal use you get only your place for sleeping. The bathroom, the kitchen, the lounge - you share them with over guests. And bathroom isn't exactly bathroom because of no bath in there - it a shower room.
The dormitories are mostly for single-sex guests. But if you are travelling in a company you may settle this question down with the hosts of the hostel and they will let all your mates of both sexes share one dormitory. Some hostels have even personal rooms, but usually even renting this one you'll have to use the shared bathroom.
The same as their guests the hostels have to economize on everything to be able to provide tourist with cheap accommodation. No breakfast included of course. You can either cook or eat out. The interior of rooms is very ascetic, the free space is very small, there very little of furniture, in the dormitories there are sometimes only beds. The room service - well on should be happy if there'll be some at all. Reminds of a summer children camp, doesn't it? Sometimes some of long-stay guests for the free accommodation do the job of the reception-desk clerks.
Hostels, one should confess, are not a very comfortable accommodation. Any hotel will be better. And more expensive otherwise. Problems among hostel guests can be plenty. Just imagine - many usually non-familiar to each other people sometimes speaking different languages have to share on bedroom. You can either make international friends or be driven into international conflicts.
One of the most unpleasant things is stealing between guests. That's why some hostels provide locking up guest belongings.
Mind that you are coming at a hostel not for comfort, but for a place to sleep. One should be seeking for the living minimum like clean and well-dried linens and working shower-cabins. You are supposed to spend all day out and return only to sleep. By the way in order that guests coming back late at night wouldn't disturb the others some hostels fix the time of last admission.
One can go on with naming those disadvantages but we should remember that it's one of the main features of youthfulness - to be very adaptive to all living conditions. Another feature that means a lot for staying at a hostel is the will to meet various people and to see the world with the least expenses.