In Thailand, people do not shake hands (although they do with foreigners) and do not kiss each other, they use the wai (ไหว้) to greet, thank and show respect.
But the Wai is not the same as a hello, because you don't necessarily do it every day to everyone.
The Basic Wai
The basic Wai consists of three movements.
The different levels of ordinary wai
In Thailand, older people and social categories are respected, so the level of wai will depend on these two factors.
Hands together at the lungs, to respond to a wai given by someone younger than you or of a lower social category:
On the chin, in response to a wai given by someone of the same age and social category:
At the nose, to be done first to older people or those of a higher social level:
On the forehead, for monks or members of the royal family:
When and how to do the Wai
As a foreigner, it doesn't matter if you don't do the wai, the Thais learn from childhood to do it and for them it's an acquired reflex.
For us, it is sometimes difficult to know when to wail, and to think about what level of clasping hands to do.
If you are struggling, you can just respond with a quick wai no higher than the chin for everyone.
An important person, in order not to lose face in public, will come and shake your hand if you do not shake it.
It's polite to respond to the wai, even the one made by a child, but it's not obligatory to respond to the wai given by the employees of a business, unless it's a person you know, they usually don't expect to be answered, they just do their job.
In both cases (children and employees), a nod and a smile are enough.
You should not wail on younger people first (except monks, patrons or royalty) or wail on older friends every day.
One wai first when dealing with or being introduced to someone older or of a higher social category.
And people respond to the wai given by people who are younger and from a lower social category.
There is no point in waving at your friends and relatives every day if you see them often, you should do so, for example, before a long journey and on your return.
It is also used to thank for help or a gift.
You have to wail the monks, even younger ones, when you want to discuss with them and they won't answer, because they are not allowed to wail on laymen, that's part of the rules they have to respect.
It is important to know the owners of the house to which you have been invited.
To go further: The wai explained in video.