D50 AutoFocus

I've bought a D50 as my first foray into the Digital SLR world - I've used a film SLR and I've used digital non-slr cameras, now it's getting serious
Please can someone help me understand the Auto-Focus switches?

My lenses are the standard kit AF-S 18-55, 3.5-5.6G ED DX and the AF-S 55-200, 4.5-5.6 ED VR DX, both of which have an Auto/Manual switch on them.
The camera body also has an Auto/Manual switch.

I switch both switches to Auto and get auto-focus without any problems but why does there have to be two switches?

The book adds confusion rather than clarification
Would appreciate any help in understanding.
Peter