The term "work" covers both physical and mental activities, and is defined as "working by exerting one's physical energy or employing one's knowledge, whether or not for wages or other benefits". Working without a valid work permit even for a day is a criminal offense. Employers who violate the law face fines of up to 60,000 Baht and/or jail terms of up to three years. Foreign employees found in breach of the law can be jailed for up to three months and/or fined up to 5,000 baht.
There are a few types of work that are reserved for Thai nationals. These are mostly related to manual labour and the production of handicrafts but also include jobs such as hairdressing. A full list can be found on the Department of Employment website.
Once you have a work permit, you can work and run a business legally, receive income from your business, open a Thai bank savings and checking account, apply for a Thai credit card and bank loans, sign up for a telephone line or mobile phone contract, import your personal belongings to Thailand, purchase a car or motorbike in your name, and apply for a one year visa extension. You will also be eligible to pay Thai income tax. However, you still cannot own land as a foreigner (see property).
Foreigners are able to work in Thailand on a one-year basis. Permits must be renewed before expiry. The validity of a work permit depends on the holder's immigration status. Holders of transit and tourist visa are not permitted to work. Unless you have a non-immigrant visa approved for by the Board of Investment (BOI), you will still have to make the obligatory visa run every 90 days.
Your employer should apply for the work permit on your behalf. Thai law places a quota on the number of foreigners allowed to work at a company based on a number of conditions. These range from ratio of Thai staff to foreigners to the amount of the company's registered capital.
In order to apply for a work permit, you must have a non-immigrant visa or a residence permit. Your work permit only allows you to work in the location which is specified on the inside cover of your work permit. If you leave those premises to work, even to meet with clients at their office, you are actually breaking the law.