Understanding PrimeFaces process/update and JSF f:ajax execute/render attributes

What exactly are process and update in PrimeFaces p:commandXxx components and execute and render in f:ajax tag?

Which works at the time of validation? What does update attribute do rather than updating value to component from back end? Do process attribute bind value to model? What exactly do @this, @parent, @all and @form in both attributes?

The example below is working fine, but I am a little confused in basic concepts.

<p:commandButton process="@parent"
                 update="@form"
                 action="#{bean.submit}" 
                 value="Submit" />