A copy-only backup in SQL Server(any version like 2012 or 2008 or 2005) is taken in cases you do not want effect the sequence of conventional SQL Server backups.
Usually in using a normal Backup,the backup changes the database and affects how later backups are restored. However, occasionally, it is useful to take a backup for a special purpose without affecting the overall backup and restore procedures for the database. copy-only backups serve this purpose.
Note: The transaction log is never truncated after a copy-only backup.
The types of copy-only backups are as follows:
Copy-only full backups (all recovery models)
Copy-only log backups (full recovery model and bulk-logged recovery model only)
Note:A copy-only full backup cannot serve as a differential base or differential backup and does not affect the differential base.
Mainly if a normal backup is taken without using copy_only then it should kept safe until next full backup finishes since a normal backup taken breaks the T-log backup chain by deleting fished transactions(Transactions written to disk already) from the .LDF File.