A VPN is a Virtual Private Network. A VPN is an alternative to a truly private network, where someone completely owns the infrastructure (i.e., wires) between locations and no one else can use it. Typically a VPN is used when some part of the network path crosses a public network such as the Internet or, in the case of wireless, "airspace." A VPN may also be used to provide additional security on private networks. The VPN builds an encrypted "tunnel" through a public/private network which provides the necessary confidentiality (this prevents snooping), sender authentication (this prevents identity spoofing), and data integrity for secure access to private resources.
A VPN client allows you to "tunnel" all of your traffic to UCSB over an encrypted, authenticated link which terminates on campus at our VPN server. The VPN server decrypts your traffic and forwards it to whatever its original destination was, but it changes the source address of your traffic from your ISP-assigned address to a UCSB-assigned address. This way, all of your traffic appears to be coming from on campus and may not be limited by internal Internet firewall rules.
For technical details specific to the UCSB VPN Service, please see the VPN FAQ page.