Thailand uses the Thai baht (symbol: ฿), code THB.
Banknotes come in denominations of 1000, 500, 100, 50 and 20 baht. Baht is subdivided into 100 satangs. Coins come in denominations of 10, 5, 2 and 1 bahts and 50, 25, 10, 5 and 1 satangs.
Coins can be hard or simply impossible to change back to euro when you leave a country. Before leaving, spend them (on postcards, a newspaper, or food or drink for the train ride), change them into banknotes, or give them away.
If you have some bahts left at the end of your trip, change it to euro at the airport or simply spend it at the airport before you fly home.