tacacs and radius

[Chad A. Adlawan]

hello everyone !
  i friend was asking me whether i can help him convert his slackware system to 
debian (since imthe only debian user in our group and their slak admin jumped 
ship) from "tacacs" to "radius" ...  whatever those are.  can someone please 
help point to me where i should look for more info regarding this ?
  also in question is for an ISP billing system, any suggestions?
TIA,
Chad