Looking for Documentation: Plugin Architecture

I will be starting a new project here soon and want to start learning what I 
need to this project. I want to create a more plug-in based architecture for 
this program where a main "processing program" calls the correct plug-in to 
do the actual processing. Then when complete return to the processing 
program to do some other processing in another plug-in, then some final 
processing in the main program. The reason for this architecture is that I 
want everything to filter through this one program, but in each case there 
could be one of an number of pre-defined functions. Also, I would like to 
design it this way so that it is easier for another developer to come in and 
create a program without having to fully know how this whole process works.

While I am not sure of the proper way to do this, I am thinking of having a 
file with the condition and the subprocedure to call. When the condition is 
hit, the subprocedure it is associated with is called. I would prefer to use 
a subprocedure for this. Would call-backs work with something like this? How 
do you do call-backs?

In reality, all I have right now is this idea in my head and no real idea on 
how to implement this. I am planning on mapping this process out in Viso. 
Here is roughly what I am trying to do. 

user calls ------> | Main | ------- calls plugin ------> | my plugin |
 | Processing | <-- calls another plugin --- | |
| Module |
| | -- calls sending plugin ---> | sending |
| | <------ Returns status ----- | plugin |

Does this make sense? Where can I read up on thoeries on this?