5512-K9: The regular ASA, for standard firewalling and VPN
5512-K9-RF: Same as above, but refurbished equipment. Its's used but comes with the same warrenty and support as "normal" ASAs.
5512-SSD120-K9: Includes the needed hardware (SSD) to install the Next-Gen Firewall and IPS-Software (FirePOWER). This gives you additional control over your data for an additional Money.