The best time of the month is when you get your paycheck! Your school (or BOE) will probably also give you a piece of paper describing it in detail. Here's an explanation for those of you wondering how everything adds up.
Salary (hoshugaku 報酬額)Edit
There are two pay scales in effect starting the 2012-2013 JET year. If you arrived in 2012 you will be on the new sliding pay scale (more years on JET more pay earned). You WILL also be responsible for ALL taxes (ignore everything about people who get out of paying taxes below). The pay scale is as follows:
3.36 million yen in their first year of appointment, approximately 3.6 million yen in their second year of appointment, approximately 3.9 million yen for their third appointment, and for those appointed for a fourth and fifth year, approximately 3.96 million yen for each year. If you arrived before 2012, you will continue to be on the old pay scale which gives you a floor (guaranteed minimum) after taxes.
Pension (koseinenkin 厚生年金)Edit
Generally around ¥20,000. On the one hand, this is a hefty sum. On the other hand, you will be able to ask for it back when you leave Japan. For more information, see The Pension Refund.
Employment Insurance (koyohoken 雇用保険)Edit
About ¥2,000. All JETs are legally required to enter (and pay for) the employment insurance scheme. Though you can't get this money back, if for some reason you are jobless and in Japan, you might qualify for unemployment benefits. For more information, see http://www.jetprogramme.org/e/current/insurance/public.html#employment
Health Insurance (kenkohoken 健康保険)Edit
About ¥10,000. Another piece of insurance that the JET is responsible for is Health Insurance. For more information, see Health Insurance
Income Tax (shotokuzei 所得税)Edit
Depending on the tax treaty your country has with Japan, you might be required to pay Japanese taxes. Fortunately, your salary will be increased to compensate you for these taxes.
Resident Tax (juminzei 住民税)Edit
Depending on your job status, nationality, and how long you have been living in Japan, you may or may not be excused from the resident tax. For example, most 1st year CIRs have to pay the tax, as do most 3rd year ALTs, but most 1st year ALTs are exempt.
Why Should I Worry About It
In most cases, your CO will handle everything and you won't need to worry about it. However, if you break contract and remain in Japan, your contracting organization will no longer be responsible for your resident tax, and you will have to pay it yourself.
Other Deductions (その他)Edit
Since we all have different contracting organizations, you might have other deductions in your paycheck. Some Contracting Organizations will deduct enkai membership dues (親睦会), and others will deduct your rent (家賃). If you have any questions about your paycheck, try talking to your supervisor.