These technical terms refer to different MRI sequences. With T1 images, fat has a high signal intensity and the spinal fluid (CSF) appears dark, having a low image intensity. With T2 images, fluid has a high signal intensity and therefore the CSF appears bright.

On both, tissue such as muscle and cartilage appear with intermediate intensity, and bone and tendons have low signal intensity.

Also, fat has high signal intensity on both, which is why it is necessary to employ techniques to suppress the signal from fat.

Note that healthy discs contain water, whereas degenerative discs, which dry out, have a low water content (and lose height).

Alert: http://www.fda.gov/cdrh/safety/neurostim.html