SSD is Solid State Disk, which instead of using a magnetic-based rotating disk with moving head, the SSD using memory chips and has no moving parts. It one day will replace traditional hard disks due to lower power consumption and much better performance, but is very expensive right now.
If you want the best performance, get an SSD. I would get the option with both drives if you are looking at the 27" and you get the advantages of large capacity for media on the hard disk, and the performance for the OS and applications on the SSD.