If there was ever a document in Thailand that has been so misunderstood - no doubt it is the Thai work permit. Some expats call it a Thai work visa which is factually incorrect. One very common problem in Thailand is the misunderstanding of a Thai work permit and what you really need to work in Thailand. Many think that obtaining a Business Visa gives you the right to work in Thailand. This is mainly due to a misunderstanding as many Thais call the business visa a work visa. It is more an issue of a mistranslation than a misunderstanding.
When you obtain a "B" visa or even an "O" visa these are used to apply for a work permit in Thailand. Hence the "B" visa gives you no rights to work or to manage a business even if it is your own, it only allows you to apply for the work permit within 3 months of entering Thailand. There are issues if you work in Thailand without a work permit.
If Immigration officers discover a foreigner working without a work permit, both the employer and the employee will be prosecuted, regardless of the contract between employer and employee. The employer will be fined between 10,000 and 100,000 baht. The employee will also be fined and deported within 30 days.
If you are looking at working in Thailand be certain that the company offering you the position will apply for your work permit once you arrive. You are not allowed to work while the work permit application is in progress as this would be working illegally. You can only work while you have the 'blue book' in hand.