Purpose of FIR Filter Structure I read Digital Signal Processing of author G.Proakis and see this FIR Filter Structure. I understand all about this but I wonder that what is purpose of classification to 4 structures? 
Here is this section:
These are just different ways to implement FIR filters. In theory these structures are all equivalent in the sense that they compute exactly the same output signal for a given input signal (under ideal circumstances, i.e., infinite precision), but in practice they behave differently. The differences are mainly the following:


their behavior when implemented with finite word-length
computational complexity
memory requirements


I'm sure that if you continue reading the book, these structures will be explained in more detail, and their relative merits will be discussed.