The Data Robotics Drobo FS features a dual core processor, with one core running the embedded proprietary OS and the other core running Linux. The unit also features five 3.5″ SATA storage bays and dual drive redundancy.
Source: TUAW
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