TACACS+ stands for Terminal Access Controller Access-Control System Plus. TACACS+ is a Cisco extension of earlier related protocols such as TACACS and XTACACS.
TACACS+ provides authentication, authorization, and accounting (AAA) services. TACACS+ provides an alternative to RADIUS. Whereas RADIUS combines authentication and authorization on a single server, TACACS+ allows the separation of these concerns onto different servers. Also, while RADIUS messages move over UDP, TACACS+ utilizes TCP. (The normal TACACS+ port number is TCP 49.)
References:
http://www.cisco.com/en/US/tech/tk59/technologies_tech_note09186a0080094e99.shtml
http://www.cisco.com/en/US/docs/ios/11_3/security/configuration/guide/sctplus.html