What is the difference between an open bite and an overjet?

Open bite: The front teeth do not touch in an open bite and overjet refers to a bite where the upper and lower teeth do come together. The space between the cheek side of the lowers and the tongue side of the uppers.

Different look.: An open bite is when the front teeth don't erupt fully and so when the top and bottom back teeth are together fully you'll see space between the upper and lower front teeth. Overjet, commonly and incorrectly called overbite is when the top front teeth are far forward of the lower front teeth when the teeth are closed together.