Thailand's currency is the Thai Baht (THB). Notes come in 20, 50, 100, 500 and 1,000 denominations; coins in 1, 2, 5 and 10 Baht plus smaller satang coins rarely used by tourists.
Cards vs cash
Hotels, malls and larger restaurants accept cards freely. Street food stalls, markets, songthaew (shared taxi) drivers and small shops are cash-only. Carrying a mix is standard practice.
ATMs
ATMs are everywhere and accept foreign cards, but nearly all charge a foreign transaction fee on top of whatever your home bank charges — withdrawing larger amounts less often reduces the total fee burden.
Typical daily costs
Costs vary hugely by travel style. A backpacker eating street food and using shared transport spends a fraction of someone staying in beachfront resorts and booking private tours daily. As a rough shape: street food meals are inexpensive, restaurant meals in tourist areas cost noticeably more, and private tours/transfers are usually the biggest single line item in a Phuket budget.
Currency exchange counters vs banks
Independent currency exchange counters in tourist areas sometimes offer better rates than airport counters or hotel exchange desks — comparing a couple of posted rates before exchanging a large amount is worth the extra few minutes.
Digital payments
Thailand's PromptPay QR-code payment system is widely used by locals but generally requires a Thai bank account to set up — not typically usable by short-term visitors, so don't expect it as a backup to cards and cash.