Yantra or Sak Yant tattooing has its origins in South East Asian animist religions and pre dates Buddhism.
Commonly seen in Thailand and Burma but less so in Cambodia the stylised calligraphy offers protection, power and other benefits to the bearer. In return they must obey specific rules relating to their chosen design. These can include never eating left over food, not walking under washing lines or slandering someones mother. A fairly mixed bag.
It is becoming increasingly popular among young monks in Cambodia, although one novice admitted to me that his somewhat western looking winged design was actually a transfer!
Whilst not essentially part of buddhism the tattooing is often carried out by a buddhist monk who uses a long needle and a personalised ink recipe of anything from palm oil to charcoal, snake venom and even (allegedly) human remains.
Here are just a couple I saw. The gentleman on the left is the charming 'Mister Nan' a senior monk at Banteay Chmar Wat. The young monk on the right has a series of spirals which represent the path to enlightenment.