The ASA-5505 is a security appliance, aka firewall/router with basic packet inspection. Base is a unit limited to an internal network, a DMZ, WAN and 10 internal devices using it to access the external WAN. The price goes up with each added feature whether it's more users, unrestricted internal networks or added security plugin card.
The difference in price on the two units you listed is because the ASA5505-BUN-K9 is 10 user and the ASA5505-UL-BUN-K9 is unlimited user.
They tend to come in 10 user, 50 user and Unlimited.
Once you buy one and get access to Cisco's support website, you find that the ASA-5505 is generically referred to as the ASA-5505 Security Appliance.
The ASA-5505 comes with the proprietary Cisco router software and ASDM management software installed.