A birdie - a bogey is one shot over par on a given hole, and a birdie is one shot under par on a given hole - so, a bogey is two shots worse than a birdie.
"Bird" was a slang term for 'good' in the 19th century and so a great shot on the golf course became known as a 'bird', which then turned into 'birdie'. It was used for any under-par score at first....