Most airlines have at least 2 classes of travel, first class and economy class. Economy class tickets are much less expensive. Each passenger more than 2 years old gets a free luggage allowance. Generally this weight limit is 20 kg of checked baggage for economy class passengers and 30 kg for first class passengers. If you check excess luggage you'll have to pay a fee calculated by the number of extra pieces and the extra weight. Every passenger is also allowed one small carry-on bag, for instance a laptop computer or small suitcase.
The rules for passengers who are traveling abroad are similar in most countries but there are, sometimes, slight differences. Here are some things to remember: if, for instance, you need to go through Customs, try to fill in the customs declaration before you talk to the customs officer. He will ask every passenger the same, routine questions about whether he is carrying anything he wants to declare: like tobacco, alcohol, presents, and sums of money.
At the check-in counter, your ticket is looked at, your things are weighed and labeled with their destination. The next formality is filling in the immigration form and going through passport control. Remember to have a black or blue pen with you because the immigration form has to be filled in block letters. You write your name, nationality, permanent address and the purpose of your trip. In most countries there is also a security check when your carry-on-luggage is inspected.
When you arrive at your destination, some of these same formalities will undoubtedly be repeated. Often while on board the plane you will be given a customs declaration and immigration form to fill in. At the airport you may be met by a specially trained dog whose job it is to make sure that no passengers are carrying any illegal drugs. In addition, the immigration officer might want to inspect your visa, your invitation and even your return ticket.