Slings (also known as Webbing) are loops of strong material used frequently in outdoor climbing.  They tend to be made out of either non-elastic nylon or Dyneema.  Slings sold by reputable climbing shops are made to a very high standard and as such are incredibly strong (the strength rating of a sling is supplied when purchasing).  

Slings are available in a variety of different sizes (and colours) and have a number of uses when climbing outdoors.  They can be used to set up belays by building anchors, can be clipped into protection to extend the reach or simply be worn over the shoulder as a method of carrying gear on.  When you decide to climb outdoors and wish to purchase your own equipment, it is advised to buy at least a couple of each size of sling initially.