Location coordinates/grid in blender - how are coordinates/locations set? I've been using blender for research purposes, i.e. matching eye movements to objects in a scene. To be sure about what I'm actually doing, I'm trying to understand as much as possible about blender. 

Here's my question: I have been working with Camera View and the Location coordinates of my sphere are (X:-6.205m, Y:-8.3m, Z:37.479cm). I've been given to understand that these Location coordinates are relative to some arbitrary origin in blender, but can anyone tell me more about this grid? where is the origin? how are locations of objects set - how does blender decide on coordinates? Is the Location of an object proportional to some other measures? 

I realize these are very basic questions, but it would really help me to have them answered. Thanks!
Here is a basic summary, Hope that can help: