Chocolate in some form has been used since as far back as 2000 BC - it was used in both food and drinks, based on the cacoa bean, by ancient South American cultures, and there are plenty of records of chocolate use in Mayan and Aztec history from at least the 6th century. The name itself seems to come from xocolatl, an Aztec word meaning 'bitter water' and then to have entered Spanish from the 15th century.
Chocolate wasn't then considered a sweet, but was used in cooking or drinks to give a strong, bitter flavour. Later the cacoa bean was used as currency in some areas.
In 1528 Hernan Cortez had the idea of mixing the bean with sugar, and this was the origin of drinking chocolate as we know it today. It spread around Europe, and in the 17th century some London coffee houses also began to use it in cakes. In the early 18th century, probably 1728, the world's first chocolate bar was created by the Fry family, a name still known today.
You can learn more about the history of chocolate here.