It depends on the airline you're flying with, the class you're travelling in, the destination and a number of other factors.
Charter & Low Cost Carriers
If meals are not included they can generally be pre-booked, at extra cost, at the time of booking. If this is the case then you will be given the option to pre-book meals during the booking process.
Most airlines sell light refreshments and snacks onboard during the flight. Drinks are almost always chargeable on charter and low cost carriers.
Depending on the route and duration of the flights, anything from a sandwich to hot meals may be provided. On most airlines, particularly when flying to long haul destinations, there is no additional charge for this service. Drinks (tea, coffee, soft drinks and alcoholic beverages) are normally available on a complementary basis although some airlines charge on certain routes.