A bulging disc is when the disc has been compressed to the point of being pushed out of it's original space and into the area where the nerves and spinal cord reside. A bulging disc may not cause any pain at all however over time and with increased natural forces or injury, it may eventually move so far as to touch nearby nerves resulting in serious pain. Pain may present suddenly however it is more likely a situation that has just been waiting to happen. You will have heard stories of how a person was pain free one day and then suddenly, as if for no reason, they developed serious and debilitating back pain.
Compression on the spine can happen naturally (with age, gravity and excess weight) or it can be caused by repetitive incorrect posture (office workers, drivers, building trades, nurses, etc) or specific injury (accidents, heavy lifting, sports).
Until the disc is compressed to the point of "bulging" out of it's intended area and touching a nerve, you may never know you have one.
A herniated disc however is where the bulging disc eventually ruptures and the inner disc material leaks out. This causes more serious pain as the gel like substance travels around the nerves and the spinal cord and is not contained within the disc.
Taking the load and pressure off the disc can encourage the disc to return to its original location. This process has been tried in many ways over decades with the most successful being traction, where the body is stretched to release pressure on the spine. In recent years this traction type effect has been introduced in the form of Spinal Decompression which is able to target the particular disc in question, rather than the whole spine, thereby providing a more gentle yet more effective result.