What is SQL Server?
SQL Server is a DBMS system provided by Microsoft. SQL Server is sometimes mistakenly referred to as SQL.

In which order do you perform an upgrade to SQL Server 2005 for replicated databases?

Distributor, Publisher, then Subscriber. You always perform an upgrade in this order: distributor, publisher, subscriber.

What the heck does ATN2 do?

The angle between the x-axis and a ray.

This is a mathematical function that returns the angle between the positive x-axis and the ray that passes through the two coordinates passed in. The angle is in radians.

How does a differential backup know which extents have changed so that it can be very quickly run?

The DCM tracks changes. The differential backup reads the extents from this structure. A differential backup uses the Differential Change Map to determine which extents have changed and need to be include in the backup. This greatly speeds the differential backup process.

What does the Queue Reader Agent do in SQL Server 2005 replication?

This agent reads the subscriber logs and moves changes back to the publisher. This agent is used when the queued update model is chosen with transactional replication. It moves changes from the subscribers back to the publishers.