When do I call it CPU and when IC-Chip?
All CPUs are ICs. Not all ICs are CPUs.

This is similar to "All cars are motor vehicles; not all motor vehicles are cars".

A CPU is an IC (or multiple ICs - see update below), which is able to do calculations based on instructions in memory, and can thus be reprogrammed. Most other ICs do one particular thing - serial in, parallel out, accellerometer, 555 timer, etc.

UPDATE - as was correctly pointed out, some CPUs are made up of multiple ICs. I had never heard of this, and I do not know if this still happens - it seems to me as a limitation of technology of days past.

I guess, in summary, a CPU is a function (i.e. is defined by what it does - follow instructions in memory) whereas IC is a technology (i.e. what it is - a circuit on a chip).