A ticket could be valid for any seat for a free seating or allocated one. Sometimes, e.g. for some train journeys, both are available, with an augmented charge for a held in reserve seat. Free seating in a train means that one may have to stand, but in e.g. a cinema it means a seat is guaranteed, just not a specific one.
Paper or card is generally used, although plastic may be used instead for hardiness. Some have a barcode or magnetic band for keeping the data stored on them, higher end ones use chips to store more data and prevent counterfeiting.
Tickets have often a perforation to divide it into two parts; one is kept by the customer and the other with the ticket checker. Whether or not one can leave and re-enter with the client part only varies. It may not be allowed to evade succeeding use of one tickets by different people, or even coincident use by giving the tickets to someone before the ticket check, but it may also be allowed, e.g. in a Theatre to buy, during a Theatre, a snack or drink before the ticket check and re-enter.
Tickets may be printed in advance, or fully or partly printed when concerned, or it may be a printed form that is completed in script (e.g. by a train conductor who does not carry a ticket machine, but just a supply of forms and a pen).