I wanted to summirize some complicated RMAN concepts, using the "11gR2 Backup and Recovery User's Guide" definitions.
Channel & Parallelism
The number of channels available for use with a device when you run a command determines whether RMAN reads from or write to this device in parallel while performing the command. When the work is done in parallel, the backup of the files is done by more than one channel. Each channel may back up more than one file, but unless a multisection backup is performed, no file is backed up by more than one channel.
The number of channels available for a device type when you run a command determines whether RMAN reads or writes in parallel. As a rule, the number of channels used in executing a command should match the number of devices accessed. Thus, for tape backups, allocate one channel for each tape drive. For disk backups, allocate one channel for each physical disk, unless you can optimize the backup for your disk subsystem architecture with multiple channels. Failing to allocate the right number of channels adversely affects RMAN performance during I/O operations.