Can you give us some context around your question .....I can certainly describe static versus dynamic but I do noy have a context for variable PDF forms.
Static - this is where you design the form and what you see is what you get. There is no restructuring or dynamic creation of objects after the form has been layed out.
Dynamic - the ability of the form to reflow itself to fit the desired content. This may mean adding additional rows in a table or simply growing a field to display all of the content within that field.
Perhaps an example woudl make it clearer. If I have a question on a form that says How many dependendants do you have? In a static form you woudl have x number of sections available for me to enter my information (lets assume 3). If I answer 1 dependant then there are 2 empty sections that take up space for nothing. In a dynamaic form there woudl be no spaces for dependants until I answered the question and if I answered 5 then the form woudl add 5 sections for me to add the dependant information. If I answered 2 it woudl add 2 sections.