A SCSI channel is a physical connector on the SCSI controller, where you connect a SCSI cable; multiple drives (or other SCSI devices) can be connected to a single SCSI channel forming a "chain" (usually ending in a SCSI terminator), and all of them will share the same physical connection and bandwidth; each device will be identified on the channel by its own SCSI ID. Usually, the maximum number of devices is 15, plus the controller itself, for a total of 16 possible IDs.
A single SCSI controller can have multiple channels; this way, it can both manage more devices and provide higher performance.
RAID is actually unrelated to channels: it's a way to logically group many physical disks in a single logical volume, providing redundancy and/or higher performance; but you can have a multi-channel SCSI controller without RAID, a single-channel RAID controller, or a multi-channel RAID controller.