In folks culture known as Yan Wang (閻王) or Yan Luo Wang (閻羅王), whereas in Taoism, they are officially known as Shi Dian Ming Wang (十殿冥王/明王) or Shi Dong Zhu (十洞主) - literally known as Lords of Nether Realm and in other contexts, such as Buddhism or Hinduism, they are being addressed as Yama Kings.
Due to influences by other Religions, there's also the existence of these Ten Lords, so till today, a lot of Folks still have some confusions on them by mixing them up or calling the wrong titles.
In Taoism, these 10 Ming Wang are actually the manifestation of the 10 Directional Lords of Salvation for the Suffering Souls (十方救苦天尊化身), which they are the 10 manifestation of Lord Tai Yi 太乙救苦天尊 (the key person in the Salvation of Suffering in Taoism).
Why is there a need for so much Ming Wang to be manifested for the looking after of the souls?
Due to the whole Universe is so big, wide and infinite dimension, Lord Tai Yi has to manifest himself into 10 other Lords in-order to salvage the million suffering souls in the whole Universe in a split second, and these is why, the 10 manifested Ming Wang are needed for this task and of cos, each of them is given other specified tasks to look into, etc.
Here's the intro on the 10 Directional Lords and Ming Wang:
東方玉寶皇上天尊(化身一殿秦廣王)
East Yu Bao Huang Shang Tian Zun
(manifest into 1st court Lord Qin Guang)
南方玄真萬福天尊(化身二殿楚江王)
South Xuan Zhen Wan Fu Tian Zun
(manifest into 2nd court Lord Chu Jiang)
西方太妙至極天尊(化身三殿宋帝王)
West Tai Miao Zhi Ju Tian Zun
(manifest into 3rd court Lord Song Di)
北方玄上玉晨天尊(化身四殿五官王)
North Xuan Shang Yu Chen Tian Zun
(manifest into 4th court Lord Wu Guan)
東北方度仙上聖天尊(化身五殿閻羅王/森羅王)
North East Du Xian Shang Sheng Tian Zun
(manifest into 5th court Lord Yan Luo/Sen Luo)
東南方好生度命天尊(化身六殿卞城王)
South East Hao Sheng Du Ming Tian Zun
(manifest into 6th court Lord Bian Cheng)
西南方太靈虛皇天尊(化身七殿泰山王)
South West Tai Ling Xu Huang Tian Zun
(manifest into 7th court Lord Tai Shan)
西北方無量太華天尊(化身八殿都市王)
North West Wu Liang Tai Hua Tian Zun
(manifest into 8th court Lord Du Shi)
上方玉虛明皇天尊(化身九殿平等王)
Upper Yu Xu Ming Huang Tian Zun
(manifest into 9th court Lord Ping Deng)
下方真皇洞神天尊(化身十殿轉輪王)
Lower Zhen Huang Dong Shen Tian Zun
(manifest into 10th court Lord Zhuan Lun)
Larger Res-Pic at http://groups.yahoo.com/group/taoism_deity_singapore_2007/photos/album/251114378/pic/list
