There are thousands of International Schools now all around the world, but they vary considerably in their quality. The better ones have a lot of money behind them and are affiliated by an international organization such as CIS (The Council of International Schools). They will teach an international curriculum and offer internationally recognized exams. An increasing number also offer the International Baccalaureate (IB). These schools require at least some prior experience working in your home countries local school and the required qualifications to teach the subjects. I would recommend any qualified teacher with at least 2 years experience who want to live and work abroad, to teach in International Schools.
TEFL at Local Schools
There are a number of countries that want Native English speakers and teachers to teach English as a Foreign Language in local state schools or private schools. This is also a great opportunity for teachers that are looking to live and work aboard looking to start a new life. The problem here is that salaries are often at the local rate and so if you have financial commitments back home it won't be enough for you on a permanent basis. You also have to be careful about the local regulations which are often less enforced compared to western state education regulations. I have heard of teachers becoming very frustrated and de-motivated through what they perceive as essential systems and procedures that are just not in place. There is of course the culture difference. Unlike in an International School you will be working predominantly with local teachers and so you must adhere to their culture or you will find it very difficult to get along with your colleagues. At International schools by contrast the majority of teachers are likely to be expats and so you will be with a similar group of people you are in your own country.
There are a growing number of companies now that are looking for native English speakers to teach students online through their software. A lot of the students are looking for extra classes in the evening. In theory, you can work from anywhere in the world if you have a good and stable internet connection, you just need to be available in the evenings and weekends. This is another good opportunity but can be intense and many have quit early on having taken on too many hours before actually realizing how difficult it can be.