What's the difference between the two flags of Scotland?

I am an American of Scottish desent. I always looked to the yellow flag with the red lion in the center as the flag of Scotland. But since my husband began to travel to Scotland on business, he tells me that it is the St. Andrews Cross (the blue flag with the white X on it) that dominates the landscape. I know that the blue flag is the basis of the official flag of Great Britain (with the flags of England, Scotland, and Wales all sort of superimposed on each other). But which flag is used in which occasions? Does it matter? If so, why is it the St. Andrews Cross that is more common? Why did Nova Scotia, (New Scotland) in Canada, take versions of both flags to create their own province flag? (They reversed the colors of the St. Andrews Cross and then used a crest of the yellow flag and red lion in the middle). I'm just curious if there is anyone who can distinguish this one for me out there!

The blue-with-white cross is the official flag. In fact, it's the oldest national flag still in use, dating back to the 9th century!

The yellow flag with the red lion is not an official flag, but it is "often incorrectly used as an alternative national flag, especially at sporting events."

You'll find a good account at the link below.