A person, object, or situation that's bizarre is very odd or unusual in appearance, style, or character. If your teacher walked into class wearing a purple boa, lime-green cowboy boots, and a scuba mask, you might comment, "How bizarre!"
Things that are bizarre often involve strange contrasts or unexpected elements. Bizarre was borrowed from French, from bizarro "angry, fierce, strange," from Spanish, "bold, like a knight." The Spanish word was probably influenced by Basque bizar "a beard," since beards were taken as signs of energy and spirit. Nowadays it takes more than a beard to get you labeled as bizarre. Unpredictable behavior is often called bizarre — like when your teacher conducts a lesson in Pig Latin.