What are the exact differences between users and readers in the active directory roles. I assume that users can modify certain things while readers are 'read-only', but i'd like to verify so that a) I don't forget any important details, and b) that my assumption is correct
There's no such thing as a Reader role in AD out of the box. Are you perhaps looking at AD LDS or ADAM? ...
OK so out of the box AD LDS/ADAM has no read permissions (unlike AD). If you're in the "Reader" role, you can see everything in that partition. If you want more granular permissions, you'll want to use something like LDP to create those ACLs.