Secure Digital cards VS Compact Flash - Difference?

Some of my family is looking to buy a new Digital Camera. The one they want says it uses "Secure Digital Cards". Is that the same as compact flash or whats the deal with them? I thought Secure Digital Cards were some kind of encrypted card so data could be safely stored, but what good would that be on a digital camera?

They already have lots of compact flash cards - 8mb, 16mb, 32mb 64mb and a 128mb and they dont really wanna have to buy all new cards again.