GeoSpatial anything gets complicated quickly - one thing that almost helps is that GeoTools is based on “standards”.
Where possible we define our library in accordance with standards published by the Open Geospatial Consortium (OGC).
We do this so you can download lots of documentation and you will find the names line up between the GeoTools code base and the class diagrams in the standards). This is an example of the don’t invent here policy - the documentation in this case is not invented here.
Recently the OGC started collaborating with ISO TC211 Standards (which you have to pay in order to read). This is EVIL at least as far as open source is concerned - one nice person (Bryce!) has made up “primers” on the ISO specs.
These primers are not kind to the the faint of heart, but they are kind to the faint of budget.
The Java Topology Suite is an implementation of the SFSQL standard:
A word of caution - none of the above standards have an “OGC Compliance Suite” that can be passed in order to receive an official stamp. When implementing a service (such as WMS or WFS) you can ask to be tested and if successful receive a badge stating that you are standards compliant; all we can do is say we implement the standards.