Check in is a part of the air line travel. This allows only those who purchased a valid and confirmed air ticket from an air line company for a particular air trip in one of their flight which had been given with a particular flight number. At the airport departure terminal or transit terminal the air line will have all the information about the passenger in their computer and it will be handled by the ground officers. They will check the passenger's information and will accept their luggage and will provide the boarding card 3 hours before the flight take off.
To make air travel easy and enjoyable most of the air lines are introducing online check In for their passengers. This will allow the passenger to arrive at the airport with minimal time and special counters for them to check in and get their boarding passes, while normal counters too accept them. Normally this facility is available for the passengers to do online check in within 24 hours from the flight time and 2 hours prior to the flight time it will close. Limited number of group passengers too can use this facility. Any children under the age of 16, and traveling alone can't use this service. Accompanied children can be checked in with the adult passenger who is accompanying.
The Online check in can be done by passengers through any computer connected to the internet by accessing the airline's web site which issued the air ticket. Fill the necessary information in the form and once it is done passenger will get the confirmation receipt which has to printed out from the computer and to be taken to the airport. This will be exchanged for the boarding pass at the airline counter. ( If the passenger can't make it, have to do the normal check in at the airport early as possible.)
When filling you have many options like changing of seat, and reviewing any special meal requests you may have made at the time of booking before finalizing your online check-in. There may be place to enter Advanced Passenger Information if required by the destination country.
Normally passengers with online check in and without baggage can arrive to the airport one hour prior to the flight time, those who need checking baggage will need 90 minutes of time.