From a teacher's point of view it is all the things we must teach for a particular examination. So for my FCE students I need to refer to the FCE syllabus, and for the CAE students to the CAE syllabus. So if someone is doing PET I need to check the syllabus so I teach them things they are likely to be asked to do in the exam. The exam board will decide on the syllabus.... which will be planned to cover listening/writing/speaking/reading/use of English. If you buy an exam textbook it will be designed to cover the syllabus. So if you are doing PET and buy a PET textbook it will train you in the exam techniques needed, and follow the PET syllabus. The actual syllabus can be very long and detailed. A good exam textbook will cover what you need to know (though it is always a good idea to read other books and talk to lots of people to increase you vocabulary and confidence!).

A syllabus is the document a teachers follows when they teach a subject.