Can anyone tell me how a program like this might work:
http://www.red-gate.com/products/SQL_Data_Compare/index.htm

I want to backup databases into a central repository but I only want
the records that have changed for that day. This program seems to do it
efficiently. Does Sql Server, Oracle, etc offer any sort of built in
way of doing this via metadata or a similar mechanism?

I want to be able to mirror databases in such a way that I dont need to
have the admin password for them. I don't want to alter the database
schema in any way.

The only way I can think off the top of my head is to assign MD5
checksum values to rows and then compare the checksums in the master
and copy, however, to do it efficiently, you'd have to save the MD5
values as you go, which would involve altering the schema on the master
table.