Thailand's unit of currency is the baht (abbreviated to "B"), which is divided into 100 satang. Notes come in B10, B20, B50, B100, B500 and B1000 denominations. Banking hours are Monday to Friday 8.30am-3.30pm, but exchange kiosks in the main tourist centres are often open till 10pm, and upmarket hotels will change money 24 hours a day. The Don Muang airport exchange counters also operate 24 hours, so there's little point arranging to buy baht before you arrive. If you have a PIN number for your credit/debit card, you should also be able to withdraw cash from hundreds of 24-hour cashpoint machines around the country.
In a country where the daily minimum wage is under B150 a day, it's hardly surprising that Western tourists find Thailand an extremely cheap place to travel. At the bottom of the scale, you could manage on a daily budget of about B300-400 if you're willing to opt for basic accommodation and eat, drink and travel as the locals do, spending B80-120 for a room (less if you share), around B100-150 on three meals, and the rest on travel and incidentals. With extras like air conditioning in rooms and on buses, taking tuk-tuks rather than buses for cross-town journeys, and a meal and a couple of beers in a more touristy restaurant, a day's outlay will rise to a minimum of B800. Staying in expensive hotels and eating in the more exclusive restaurants, you should be able to live in extreme comfort for around B2000 a day.