Someone or something joyous is characterized by joy: extreme, exuberant, deep happiness. If you won the lottery, you’d probably feel joyous.
Joyous is a strong word for the absolute best moments in life. Weddings and graduations are usually described as joyous occasions. You would feel joyous if you accomplished a lifelong goal, such as earning a gold medal in the Olympics. In other words, joyous goes beyond the simple feeling of happiness you might get from watching your favorite TV show or eating a delicious sandwich — unless of course it’s a really amazing sandwich.