The disc is the shock absorber of the spine.  It's not like a piece of rubber that can slip, but rather a gel filled sac.  The covering has many layers like an onion that can get torn or injured.  The outer portion of the disc can cause pain.  When only some of the outer fibers are torn, it can cause a disc bulge.  When all the fibers are torn, the inner gel squirts out.  This is called a herniated disc.  The best test to look at the disc is an MRI.