Well 10-96 beat me to it and did a fine job too but I typed and typed on my reply so I'm posting it anyway.
Okay, I’ll step up and state the really obvious points (cause that’s all I know) and then let the more knowledgeable folk here fill in the subtleties of the devices.
Moon clips:
https://www.google.com/search?q=moo...Qk_W6m9PM:&usg=__YjPMy0ECZfjJEx8DIKPAK5eq9Rg=
Moon clips are for revolvers with swing out cylinders or top break cylinders that allow them to eject rimless cartridges.
The most popular moon clips are for 9mm and .45 ACP.
Moon clips are faster than speed loaders. Watch Jerry Miculek on youtube use them.
Revolvers CAN load and fire rimless cartridges if the cylinder is machined so that the cartridges headspace on the case mouth, that is, the cartridge drops into the cylinder and the case mouth hits the slightly smaller diameter part of the cylinder. Now the cartridge cannot go farther. When the cylinder is closed and the gun fired the firing pin hits the primer the cartridge will go off. Without the moon clips though you have to use something to poke the empty cases out of the cylinder. The star extractor has nothing to push against since the rimless cartridges have no rims.
The moon clips hold the cartridges. There are full moon clips and half moon clips.
There have been some clever revolver designs that do eject rimless cartridges but they aren’t very common.
There have been some modifications where spring wire is put around the star extractor so that the revolver can eject rimless cartridges but again they aren’t very common.
Speed loaders can drop cartridges into a cylinder but they don’t help with extracting the fired cases from the cyclinder.