Dyneema ropes are made from Ultra High Molecular Weight Polyethylene, or UHMWPE fibers. Dyneema offers by far the best strength to weight ratio of any material used for ropes. You will find high grade Dyneema as the core of most high performance lines. Besides the strength to weight ratio Dyneema also has proven to be abrasion resistant, resistant to UV light and does not absorb water so it floats and stays light when wet.
Dyneema is 10/15 times stronger than steel wire, and often 8 times lighter. It has become the go to line for sailboat running rigging. It has been around for 20 years but prices have dropped to the point where Dyneema lines can now be used on non-racing boats.
A good example of this is the new Viper rope from New England Ropes. This line uses a lower end SK38 dyneema core, which is still better than any non dyneema core and much better than Dacron, or Polyester, Viper is priced to be competitive with VPC and Sta-Set and other standard running rigging lines.