Seagull was put together with the developer in mind, but to a certain extent it can also be used "out of the box" by non-technical users.
The wiki is the best source for developer information, but for starters you can take a look at
The following diagrams give an overview of Seagull design:
In the left margin listed are Packages, Files and Classes. For maximum readability forget about files and go straight to the packages
Seagull uses the PEAR library almost exclusively for library classes. Current packages being used are: