休憩 きゅうけい ： break at work (昼休み is actually a type of きゅうけい） OR break from studying/training. Just long enough to eat some food and stretch - usually 15 minutes to an hour.
休み やすみ： a day off from school or work.
休日：きゅうじつ； A holiday - Christmas, Arbor Day, etc.
休暇：きゅうか： long vacation from work (maternity leave, 2-week vacation to Barbados, etc.)