Twice a year (Fort he time being in July and December) Japanese Ministry of Education holds Japanese language level determining exams called “Japanese Language Proficiency Test (JLPT)) (Nihongo Nouryoku Shiken-日本語能力試験) simultaneously all over the world. These exams like TOEFL, IELTS etc. gives you reliable credit to prove your Japanese level.
You can take advantage of that credit in various areas like finding job, studying abroad, getting promotion etc.
Starting from N5 (Beginner Level) until N1 (Advanced Level) it has 5 categories. For all levels needed kanjis and words vary. For instance N5 level may require 100-120 kanjis and more or less 800-1000 words to be known.
There are several sourcing books to study for each part of the tests. Applying for JLPT must be executed some time before the tests (2-3 months earlier).
This is only general information for JLPT. Details should be discovered.
When you get ahead in learning Japanese, keep in mind to get a certificate reflecting your level.
Self-Study Material :
Here is a useful ebook for JLPT N5 studying.