The HSK, Hànyǔ Shuǐpíng Kǎoshì or Chinese Proficiency Test, is China's only standardized test of Standard Chinese language proficiency for non-native speakers such as foreign students and overseas Chinese.
The Hànyǔ Shuǐpíng Kǎoshì is divided into 6 levels of difficulty, with each level building on the vocabulary and knowlege from the previous one.
Each year HSK certificates are issued to those who meet required scores. The HSK is administered in Simplified Chinese. In addition test takers with outstanding results can win a scholarship for short-term language study in China.
The test is administered by Hanban, a non-government organization affiliated with the Ministry of Education of the People's Republic of China.
The HSK test approximates the English TOEFL, and an HSK certificate is valid without any limitation in China. The test aims to be a certificate of language proficiency for higher educational and professional purposes.
This page contains a list of all the new characters introduced at each level of the HSK.
This details each vocabulary term introduced at each level of the HSK.
There are several places online that allow you to access mock HSK exams, however personally I would recommend either Hanban's chinesetest.cn or Confucius Institue for Business London.
There are designated test centers for the HSK both in China, and many other countries around the world. A list of test centers can be found at the official HSK website.