What's the difference between a regular backup and a copy only backup?

A copy only backup does not interupt the backup chain when you are doing multiple types of backups such as full, differential and log backups.

If the database is set to SIMPLE recovery it won't make a difference if you use the copy_only option or not.

If the database is set to FULL or BULK_LOGGED then if you do a copy_only backup for a full database backup this will not interupt the backup chain for your differentials. If you do a copy_only backup for a transaction log backup this does not interupt your backup chaing for your transaction log backups. Doing a copy_only for differential backups does not make a difference.

If your database is in SIMPLE and you are doing full and differential backups a copy_only backup does not interupt the next differential. A differential backup is a backup of all extents that have changed since the last full backup.

You can refer to this tip to get some additional info: http://www.mssqltips.com/tip.asp?tip=1772