Well, I'll see if I can give a stab at an explanation for you. You might already know some of this, and I'm going to do a simplified, generic version. Most(not all) Allen Bradly plc's will follow this general guidline, but if you are doing more advanced programming you'll need to do some further research on your specific processors.
*DISCLAIMER: THIS IS NOT HOW THE PLC WORKS... THIS IS A GENERIC OVERVIEW OF HOW SOME PLC'S WORK, AND IS NOT TO BE MISTAKEN FOR ANYTHING OTHER THAN AN ATTEMPT TO EXPLAIN BASIC CONCEPTS. THERE IS A LOT MORE GOING ON THAN I AM TALKING ABOUT HERE*
The plc works on what's called a "scan cycle", which is broken up into 3 basic sections. You will have an input section, a calculation section, and an output section.
1. The input section reads the current status of all information being used in the program(input values, current data table values, etc) and stores them for this 1 cycle
2. Based on the values being stored for this 1 cycle, the plc calculates what all actions need to be taken(outputs turned on, counters incremented, comparisions used, etc)
3. The plc implements all of the things that needed to be done, based on the calculation section.
Phil has an illustration that might make a little more sense
here.
So... The plc will take a "snapshot" of all input states, counter values, etc..... The plc will then figure out which outputs and bits need to be turned on(based on the snapshot values), and then the plc will turn on outputs, bits, etc. based on the evaluation. And then the plc will do it over...and over...and over....etc.
Hopefully that makes sense, and is not just confusing you more