The DroboElite features 8 drive bays, dual gigabit Ethernet, dual-drive redundancy, and support up to 256 virtual drives.
Source: HotHardware
The Drobo S features 5 bays, dual-drive redundancy and eSATA connectivity. It also sports a faster processor compared to the standard Drobo model.
Source: HotHardware
The Drobo Pro features 8 drive capacity with dual Firewire 800, USB 2.0, and iSCSI. At the moment it allows for up to 16TB of storage and is able to be rack mountable.
Source: CrunchGear