Scheduled airlines, like the major flag carriers, operate flights according to schedules drawn up to cover several months or even years at a time. These airlines are thus committed to flying the planes for the said periods on the said routes whether they are filled or not. In so doing they offer a predictable range of services and can thus be booked up to 1 year in advance in some cases. Also, they nowadays often operate in codeshares with other carriers to offer even more services but, to do so, the importance of keeping to schedules is emphasized. Scheduled flights are thus preferred by most travellers, especially business flyers.
Charter airlines charge lower fares by anticipating a full load or else pre-selling the plane's capacity to an agent, who then sells off the seats to passengers. In order to do so, however, they must be assured of a full load and so they generally operate only at peak periods on specific routes. For example, the Caribbean is served by a number of European charters during the peak tourist season, November to April. Some of these carriers include Condor Flugdienst, jmc air, Airtours, Britannia and Martinair. In POS, however, the charters usually come from JFK or YYZ during Carnival (February), summer and Christmas when passenger traffic here is heaviest (eg Tower, ATA, Royal, Skyservice, Air Transat). By preselling the seats and anticipating demand charters are able to serve long, thin routes which would be very unprofitable to run on a scheduled basis.
Hope that this helps.