What is the difference between IEC 61511 and IEC 61508

A lot of engineers and functional safety practitioners struggle to understand the difference between the two most popular standards for functional safety used in the industry as at today. A nicely written article by John Yozallinas CFSE recently published on the Exida website perhaps explain these differences  as clearly as I have ever seen any one explain it. according to John,

"IEC 61508 is considered a basic or “umbrella” standard for functional safety.  It is generic and sometimes even vague.  IEC 61508 was intended that various industry sectors provide their own specific standards and guidelines as needed.  Here we can see the relationship between 61508 and other standards, such as 61513 for the nuclear sector, 62061 for the machine safety sector, and 61511 for the process control sector."

Source: http://www.exida.com/Blog/functional-safety-standards-iec-61508-vs.-iec-61511