SV2AGW's software TNCs and TNC management layers (AGWPE and AGWPE Pro)

A hardware TNC is a physical device that has transceiver facing analog i/o and PTT (push to talk) control facilities on one side, and typically, computer facing RS-232 (serial) data connection on the other.
 
Hardware TNC's typically employ specialized chips to convert the audio waveform into the digital data it represents.  With enough CPU power, this can be done via software, which is what George Rossopoylos (SV2AGW) has done with AGWPE (the free version) and AGWPE Pro (the payware version).
 
The older TNC's used to be interfaced directly with dumb terminals, but now-days, computers are more the norm.  And it's a good thing too, since, with AX.25, it's protocols support the notion of multiple distinct conversations happening in an interwoven way, which would have been way too hard to keep track of by hand, using a dumb terminal.