Cable rails are essentially a series of metal wires running horizontally between rail posts.  This system works well to maintain open views and yet allow ventilation as opposed to using glass. 

Cable rails must be installed under very tight tension to meet IRC requirements.  One drawback to horizontal rail systems as that they do create a ladder effect that small children can climb.  There are a variety of systems available offering different hardware connectors for attachment.  Some systems include posts and top caps to complete the design others can be used with standard wood rail parts. 

Stainless steel is commonly used for cable rail to prevent corrosion.  Cable rails offer an industrial aesthetic that may compliment a modern style home but may look out of place on some traditional style houses.