Notes and Coins
Since 1902 the money in circulation in Thailand has been the Thai baht. In Bangkok, you'll be able to find coins of 25 and 50 stang, and of 1, 2, 5 and 10 baht. On the other side of the coin, there are normally impressions of Buddhist temples.
There are also bills of 20, 50, 100, 500 and 1,000 baht. A picture of the king, Rama X is on all of the bills.
Cash or Card?
Bangkok is the Thai capital, and one of Aisa's most cosmopolitan cities, meaning that you'll have no problem paying by card in the majority of shops and restaurants. However, this is not so common in the street food stalls or in the outskirts of Bangkok. It's also possible that in some shops they'll charge a small commission to pay by card, so it's always best to ask.
The best way to get your baht is to change money at home. Another option is to take money directly from the ATMs once you arrive, although Thai ATMs charge a fee of 220 baht to take out money (about US$7)
In Bangkok, you'll also find plenty of bureaus de change, which offer pretty reasonable rates.
US$1 is worth approximately 31 baht.