Haha. Depends on who you ask. Some people are really strict about it and say that it's a story with exactly 100 words; others, like myself, count a drabble as any story with between 100-500 words.
They're really fun to write and read. You should create some. <3
A drabble is a very short story, specifically a story that is exactly 100 words, but there is a leeway area of 75-125 words. Some people consider anything under 500 a drabble.