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
When you need to replace cartridge it's easier with removable headshell.
There are also upgrade options for better headshell. Having more than one headshell gives you advantage to mount fast cartridges with them for certaine purposes you may entertain.
Phono cartridges sound different! I have a moving magnet (mm) mounted on one headshell and a Moving coil (MC) mounted on another. Each, provides something different to the musical presentation. Being able to switch cartridges quick and easy is a plus. How you could have a TT with multiple tone arms but that gets pricy.
Wow this question is to tough.
It adds some weight to the Arm for better tracking
Ok , if you trade out headshells with cartridges mounted in them , is there a new set up procedure required ? Or do you just remove one , install another and play records ?

Thank you
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.
Except in the unlikely event that the two cartridges need the same VTF, antiskate and arm height settings, those will have to be adusted with each cartridge change.

Additionally, much depends on how accurately the headshell/tonearm connection was machined. If it puts the cartridge in *exactly* the same position each time its removed and replaced, great. Of course no mechnical interface is perfect. To whatever degree there's room for variation, you would need to re-check overhang, zenith angle and azimuth.
If you plan to have 2 different cartridges with same VTA and VTF when mounted, than you just swap'm. If you plan to use another cartridge let's say for 78rpm records, regardless of VTA and VTF it probably wouldn't matter
At a minimum, you're generally going to have to adjust vertical tracking force for each cartridge, and usually anti-skating as well. Most cartridge have their own individual sweet spot within the recommende tracking force range.

Not mentioned up to this point, but it's waaaay easier to mount a cartridge on a detachable headshell AND significantly lower the risk of damaging the cantilever during the process.
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
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 .