The IEC (the International Electrotechnical Commission) is an international standards organization that publishes electrical and electronic standards. Hundreds of them, in fact; standardizing everything from letter symbols in electronics to medical equipment and railroad communication and signaling.
Specifically, IEC 61131-3 is the standard for “Industrial-process measurement and control – programmable controllers.” It sets standards for PLC programming languages and the programming interface itself. It specifies the syntax and semantics of PLC programming languages; specifically, the graphical languages of Ladder Logic and Functional Block Diagrams and textual languages of Instruction Lists and Structured Text, and Sequential Function Charts as well.
The latest version (61131-3) was published in 2013. It includes technical changes related to data and programming of PLC languages and specifies new data types as well as object-oriented features of classes and function blocks.
As for PLCopen, this is an independent organization focused on industrial controls. It is vendor and product independent and accepts memberships from PLC and automation manufacturers. Its core activity is centered on making software development for PLCs more efficient. Some of the ways it does so is by working to encourage reusability of code as well as offering libraries of control-specific functions to PLC programmers, all of which is based on standard, existing tools that are readily accessible. With the emphasis on PLC programming, the group’s work is naturally centered on the IEC 61131-3 standard for programmable controller design.