More often than not, vanilla extract may be used in place of vanilla bean. The difference matters most when vanilla is the dominant flavor in your dish or baked good. When that's the case, you might want to opt for the clean, pure flavor (not to mention the appealing little specks) of vanilla bean. Vanilla extract, in comparison, can have a slight edge to the flavor and aroma, a result of the alcohol used to make it.
When using extract in place of vanilla bean in a recipe, use 1 teaspoon for every one inch of vanilla bean. Be sure to replace vanilla bean with vanilla extract and not vanilla flavoring or imitation vanilla, which are both a far cry from real vanilla.
For more ways to use vanilla, see 7 Ways with Vanilla.