/dev/disk1 is the raw block device. /dev/disk1s1 is a partition of the device
if it had more partitions you would also see /dev/disk1s2 and disk1s3 and so on.
You would use /dev/disk1sN (N is the partition you are interested in) if you are going to manually mount or unmount a partition.
You would use /dev/disk1 if you are going to dump something to or from the device.
For example, you could
sudo dd bs=8k if=/somefile.iso of=/dev/rdisk1
dd bs=8k if=/dev/rdisk1 of=./image_of_drive.iso
The /dev/rdisk* I think are buffered versions of /dev/disk* I am not sure, but you can read or write to disk1 or rdisk1 interchangeably