* What is Thailand Non-Immigrant Visa?

According to Thai Law, a foreigner must apply for a Non-Immigrant Visa when he/she wants to stay or work in Thailand (other than tourism purpose).
The Non-immigrant visa is of special importance, being the only visa for foreigners temporarily staying in the kingdom that entitles them to apply for work permits. Approval or rejection of application for work permits is done according to the 1978 foreign labour act, section 11, which states that

"foreigners applying for a work permit according to section 7 must be residents of the kingdom or have received permission to temporarily stay in the kingdom according to the immigration laws. They must not have received permission to stay in the kingdom merely as tourists or on transit..."

Non-Immigrant Visa has several categories depending on the purpose of your visit:

diplomatic visa (D) for those employed by an embassy,
a business visa (B) for those who want to conduct business in Thailand
a retirement for those who want to retire and stay in Thailand
a mass media visa (M) for accredited business or press representatives,
a dependent visa (O),
an expert visa (EX) for those performing skilled or expert work,
an investor visa (IM) for foreigners who set-up their companies under the Board Of Investment BOI
a study/education visa (ED) for teachers.
Official (F). For Performance of official duties (involving the Thai government),
Capital Investment (IM).
Investment (with concurrence of the ministries and departments concerned). BOI (IB).
Investment subject to the provision of the laws on investment promotion,
(ED) for Educational study or observation.
(R) For Missionary work.
(RS) For Scientific research or training, or study in an educational institution in the Kingdom.
(EX) For Performance of skilled or expert work, and
(S) For Participation in an officially recognized sports event
Others. Including dependents and retired persons.