Ladder Diagram (LD or RLL) is a widely spread graphical programming language for PLCs. It is a programmatically replacement for relays and coils which can be combined using AND and OR operations. It is best suited to compute Boolean expressions.
You may connect relays, coils and function blocks freely. When required you can choose to have additional EN/ENO inputs and outputs.
Most editing is done by drag and drop. The picture shows the drop targets as a visual feedback while inserting a new ladder diagram element. Depending on the drop target position the new element can be ANDed, ORed or start a new rung.
Similar drop targets are displayed while moving or copying one or more elements of the ladder diagram.
IEC 61131 -3 Ladder Diagrams can also be used in Sequential Function Charts (SFC) to define the content of actions and transitions.