The JLPT, or Japanese Language Proficiency Test, is considered the standard exam to take for Japanese learners and its qualifications are more widely recognised. The test has been around since 1984 and tests vocabulary, grammar, reading and listening. If taking the test in Japan, there are two opportunities to take the exam each year - once in July and another in December. If taking the test elsewhere in the world, the frequency will either be twice a year or once a year only in either July or December.
The J Test, or the Test of Practical Japanese, started in 1991 and its levels are divided into beginner, beginner to intermediate and advanced. It tests reading, listening, writing and vocabulary. The exam is offered six times a year for intermediate to advanced learners in Japan and three times a year for beginners learning in Japan. This test is not administered globally - there are a select locations around Asia and Brazil where you can take this exam if you are unable to take it in Japan.