Activity Diagrams are graphical representations of behaviors (aka ‘activities’) that are linked together in some way to achieve some sort of result. Proper “Activity Diagrams” are a UML construct, that similar to IDEF3 Process models. They borrow several conventions from ANSI/ISO’s spec for Flow Charts - including diamonds for decision nodes.
Example from Wikipedia
I find UML Activity Diagrams to be somewhat confusing, the “bar” notation for representing parallel splits & joins is too visually distracting.