Magnesium Citrate vs. Magnesium Oxide

What is the difference in Magnesium Citrate and Magnesium Oxide? I would like to try using magnesium to solve my chronic severe constipation problem, but I am not sure what kind of magnesium to use. I tried magnesium years ago and it worked for a while and then did not. I am wondering if I purchased the wrong kind back then. Thanks for your help!