http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/admin/8_0_1/ccmsys/a08cti.html
A program called CTIManager includes the CTI components that interface with the applications that are separated out of Cisco Unified Communications Manager. The CTIManager service communicates with Cisco Unified Communications Manager by using the Cisco Unified Communications Manager communication framework, System Distribution Layer (SDL). Installation of the CTIManager program occurs on the Cisco Unified Communications Manager server during the Cisco Unified Communications Manager installation. You can have one or more CTIManagers active in a cluster, but only one CTIManager can exist on an individual server. An application (JTAPI/TAPI) can have simultaneous connections to multiple CTIManagers; however, an application can use only one connection at a time to open a device with media termination.
It's used to integrate other servers to CUCM, such as contact center (UCCX), Emergency Responder (CER). The short summary is it allows external applications to hook into call manager by creating "fake phones" which are CTI ports as well as send calls to an application via CTI Route Points.
