The four slides below summarize how geometry works on the iPhone. It takes a
little getting used to, but it is important to understand these concepts so you
can control where images, layers, etc. will appear.
You may also find Apple's own description of these concepts to be helpful:
View Architecture and Geometry
However, you'll need to be a registered Apple iPhone developer to have access
to this documentation. It's free and pretty quick to sign up. Signing up
is not the same as getting a development certificate (which costs $99).