The difference between a CT scan and a MRI?

My friend is having some problems in her kidney; she was advised to do a CT scan or a MRI to check what the problem is. I would just like to know what the difference among the two is and which is more appropriate imaging device for this situation?

I know that MRI is a more advanced diagnostic imagery than CT scan. It uses magnets and radio frequencies to create a highly detailed series of images of the inner body. This can be used in examining tissues of your inner body.