terminal access controller access control system
remote access dial in user service
security enabled by aaa
authentication, authorization, accounting
provides a means of validation of users gaining access to a 
router 
accounting not used with radius.

Terminal Access Controller Access-Control System (TACACS) is
a remote authentication protocol that is used to communicate
with an authentication server commonly used in UNIX
networks. TACACS allows a remote access server to
communicate with an authentication server in order to
determine if the user has access to the network.

But RADIUS is also remote authentication protocol which is
used in WINDOWS, and working between RAS (Remote access
server) and IAS (Internet Authentication Server).

TACACS : Terminal Access Control Access control Server

TACACS Vs Radius

1) TACACS is an implementation of CISCO [proprietory]
   Radius in non proprietory

2) AAA [Authentication,Authorization,Accounting] can be 
handled separately in TACACS
  Radius does Authentication+Authorization in a single step 
& can perform Accounting separately.

3) TACACS uses TCP [Forgot the port numbers]
   Radius uses UDP

These 3 major differences are expected at the least ! i 
hope so !