SQL Server backup versus third party snapshot based backup using VSS
SQL Server uses checkpoint, log sequence numbers, and the transaction log to guarantee that a full backup is restored to a transactionally-consistent state, as of a particular point in time.

The exact point in time for a full database backup is the end of the data-reading portion of the backup process. To recover to an arbitrary point in time requires separate transaction log backups as well.

Online backup means the database remains accessible (and writeable) for other users while the backup is in progress. Enough of the transaction log is saved with the backed up database pages to enable recovery to bring the whole database to a transactionally consistent state when restored.

The basic idea is that pages saved to the backup are already consistent to the target point in time, or can be made so by undoing or redoing transactions from the included portion of the transaction log.

For full details see the TechNet article Understanding SQL Server Backups by Paul S. Randal.