Hi There (I Know Im late to the party as usual, so I'll give my input )
I do PLC programming and such for industrial machinery. Basically I am told that a certain machine must complete a certain task. Lets use printing for an example. Well what kind of printing determines a bunch of other stuff. Anyway. Depending on the needs of the customer a PLC is speced out (Allen bradly, Mitsubishi, Xycom, Red Lion..etc). Now the PLC is what does the work. Its the brain of the machine. Its output depends on certain sensors (either on or off, there are analog sensors as well) that get processed and then the PLC does something like switch a output on or off. Now the control panel is whats available to the end user, ie the HMI, buttons, etc. The HMI "talks" with the PLC to check on the status's of certain registers. When a HMI is programmed you can say that certain things are buttons or indicators or anything in between. You can also still have a button interface with no screen as well.
Wiring is done via a schematic. We just use different symbols.