What is the difference between Clamav and Clamtk?

ClamAV is the scanner or deamon.

ClamAV can only detect viruses and move any infected files into a quarantined folder; it cannot remove them from files. ClamAv detects viruses on all platforms, but it is primarily useful for Windows viruses and malware.

ClamTk is a GUI front-end for ClamAV using gtk2-perl.

It is common practice in Linux to split the frontend/GUI from the actual program so you can use it on a server install.

Clamtk is just a graphical interface to clamav.

apt-cache show clamtk

(...)

Description: graphical front-end for ClamAV
ClamTk is a GUI front-end for ClamAV using perl-Gtk2.
Homepage: http://clamtk.sourceforge.net