Thailand is riddled with ghosts and spirits, or "phii" (rising tone), some are good, some are bad. Some of these ghosts are the souls of death people which have not yet passed to the afterlife, but often 'phii' will refer to what we in the West may call spirits. These spirits are more unique entities, rather than the 'remains' of people, and are usually associated with something, a tree, a rock, an animal, etc.
For example, "phii khluay taa nii" is a spirit of a beautiful woman in a Thai dress that lives inside a banana tree and who comes out at night. Men seeing her will run over to hug her, but she runs back into the tree and the guy ends up hugging the tree.
Also, when nature calls and there is no alternative to using the countryside, try avoiding the old, big trees. Apologize to the spirit and explain that 'you are sorry, but that there is no alternative'.
Not all the spirits are so benevolent though; "phii pop" are evil spirits who live among the people. During the day, they seem like a normal person, but at night they go in search of food (they are said to love chicken). While out searching their body stays in bed, but they take their head with them. They tend to avoid people, but may use clothes that are outside to wipe the blood of their mouth. If a person would boil this cloth, the spirit will come running over because it hurts their stomach. They will then do anything you want to make you stop boiling the cloth.
Another ghost is referred to as the "widow ghost". She supposedly searches for a husband and sometimes takes men while they are sleeping. When men die in there sleep it is sometimes attributed to this widow ghost taking them.
To fool the ghost, men who have been scared by a suspicious death, may resort to disguising themselves as women.
In the old days almost everything in the daily life of Thai people was dominated by these spirit believes, even the architecture and direction of houses. For example, banyan trees were never planted next to houses, as they are said to be easily possessed by spirits. Broken Buddha figurines are said to be unlucky, and are disposed under a banyan tree in a temple. Look at many banyan trees, they are all around Thailand dressed in some cloth, incense is placed under them. All of that is done to placate the spirit housed in the tree.
This is just one of the many rules for a wide variety of different plants.
Nowadays, though, these kinds of spirits, both good and evil, are not really common anymore, certainly not in the cities.
Ghosts of dead people are more still very common though. It takes 3 days before a person knows he or she is dead. The ghost will then return home to collect the clothes and other personal belongings he/she needs for the afterlife. They will also talk to the living, but the living can neither see nor hear them. Dogs, on the other hand, are said to be able to see them and it is said that when a dog howls he is seeing ghosts.
These ghosts are almost always benign.
A very famous example of a ghost returning to the living is the story of Ngarn Ngak. Ngarn Ngak is the ghost of a woman who died while her husband was away fighting a war. Upon his return she continues taking care of him, her husband does not know. Other people however do, and they try to tell him, but she scares away all of them. (For a more detailed story check: http://www.thaifolk.com)
Her body is said to be laid to rest at Wat Mahabut, in Prakanong, Bangkok, and it attracts a lot of visitors every day.