What’s the difference between depression and being manic depressive?

Can you answer Kewl Katt’s question about Depression?:

I know this person who says she suffers from depression, but she has these extreme highs and lows. She goes from being giddy, compulsive and promiscuous to threatening to kill herself. Do you think she’s really manic depressive, and why would she lie about her condition?

Beck Depression Inventory