A slipped disc can also be known as a herniated, ruptured or prolapsed disc but are basically the same thing.
A slipped disc is the most commonest type of back pain and can be caused by something as simple as poor posture. Other things that can slip a disc are jarring it in a fall or accident or lifting something incorrectly (ie not bending your knees when you pick up something heavy).
What happens when the disc slips is that the jelly like centres which supports the weight is forced through the outer rings of the disc due to the excessive pressure of your spine.
This can happen when your sitting or standing for long periods, with your lumber spine flattened, instead of maintaining it with the correct curve. Lifting a weight by bending forwards can also put strain on the front of the disc which can then also force the slipped disc to damage the ligaments of your spine, facet joints or your vertebrae and nearby muscles.
The slipped disc then presses on your nerve roots which then give you pain and muscle spasm. You can also feel pins and needles along the path of the nerve which could be in your buttock, thigh or foot.