Why Does It Show 1 macintosh hd as a logical drive and another that says core storage physical volume

I am trying to figure out why I have 2 Macintosh HD Drives Appearing in disk utility. One of them i can mount and shows it as being a logical drive and the second one i can't mount but it says it has no disk errors. also shows it as being a core storage physical drive, and has the same amount of gibbets as the other but has no data on it. please help