A leotard is a skin tight article of clothing that covers the body much like a one piece bathing suit does. One of their oldest functions as an article of clothing is in ballet and figure skating. The clothing needs to be tight but pliable so that person can have complete freedom of movement. They are now also used in gymnastics where the patterns on the leotards themselves can be very ornate; though their function remains the same.
Leotards are always a single piece of clothing though do not always have straps that go over the shoulders and around the neck; they may stop at the bustline in the same way a strapless dress does. When this is done in sports a nude fabric is used to go up and around the neck/shoulders to ensure that the leotard doesn't fall off; this can also allow the effective use of having elbowless gloves on which are connected by the nude color fabric while still using only the leotard as the single article of body clothing.
While there could be an endless debate on the topic, from a girl's perspective any bunnysuit is also a leotard. With a bunnysuit the top portion is built more like a strapless dress and largely uses the bust to keep it in place. In cases where the bust size is too small to do so underwires can be used to keep the leotards up. In all other respects they are put on in the same manner and are also skin tight; with just minor alterations on how the top portion is finished.