[R] AlgDesign--exact and approximate design

[sun]

Hi
  I am trying to generate a design using Algdesign and came across terms of 
"exact design" and "approxiamte theory design", I did not find a reference 
to explain what they are, could some one shed some light about this on me?

  Another question is, I want to measure the main effects and at least two 
interactions, variables are factors, how do I ensure this in  formula, 
~.+v1:v2 +v3:v4 ?

  Thanks