Serial Cards vs. Terminal Server

[Jun G]

Hello. Just a few 'wonder questions' which I hope would be answered here.
How are serial cards different from terminal servers? What is Radius? Is it
possible to configure your ordinary serial cards as terminal servers (or
vise-versa)? If I want to have remote users dialing into my server, should
I get a serial card or a terminal server? What's the maximum speed for
these devices?

Thanks.