SQL Server Backup and Restore - Simple Talk

Chapter 1: Basics of Backup and Restore
If that size of potential loss is unacceptable, then you'll need to either take more
frequent full backups (often not logistically viable, especially for large databases) or
take transaction log backups and, optionally, some differential database backups, in order
to minimize the risk of data loss. A full database backup serves as the base for any subsequent
differential database backup.
Copy-only full backups
There is a special type of full backup, known as a copy-only full backup, which exists independently of
the sequence of backup files required to restore a database, and cannot act as the base for differential
database backups. This topic is discussed in more detail in Chapter 3, Full Database Backups.
Differential database backups
The differential backup is very similar to the full backup, in that it contains a record of
the objects and data contained within the data file or files, but the differential backup file
will contain only the data that has been changed since the last full backup was taken.
This means that a full database backup must be taken before a differential database
backup can be performed, otherwise the system will not have a way of knowing what data
has been modified. This full backup is known as the base of the differential.
If you're interested to know how SQL Server knows which data has changed, it works
like this: for all of the data extents in the database the server will keep a bitmap page that
contains a bit for each separate extent (an extent is simply a collection of consecutive
pages stored in your database file; eight of them, to be exact). If the bit is set to 1, it means
that the data in one or more of the pages in the extent has been modified since the base
backup, and those eight pages will be included in the differential backup. If the bit for a
given extent is still 0, the system knows that it doesn't need to include that set of data in
the differential backup file.
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