What is a Headshell and what does it do ?

Trying learn a few things here .
I see removable headshells and people talking about changing cartridges with them . Why ?

Thank you
Johnny53 ;
Is a "detachable" headshell different than what has been discussed so far ? Would it still be required to check the angles as mentioned above when swapping two of them around ?

Thank you
note that on some headshells there's the screw hole to mount extra weight. all to match one cartridge to another
You must set up each cart/headshell with a proper protractor (mint, geodesic, or various online printout), VTA, and tracking force. You will then be able to swap out cart/headshells with only a VTF & VTA adjustment ~ and sometimes with only a tracking force adjustment.
This is fairly obvious, but even if two different cartridges require the same tracking force they both will likely have a different self mass. In that case you still need to adjust VTF for each one. Heck you could have two of the same cart and they still may need a slightly different setup.

For best performance all the parameters should be checked and readjusted if necessary. This can be speed up if you take note of all the settings before removal, and write them down for next time. The only time a removable head shells saves you is the time it takes to remove the old cart and install the new one. About 2 mins. max.
Ok , thanks to all of you .
I have learned something here .