what's the difference: merge, 3D merge and p Merge??

i kinda get this a little but please correct me:
merge- 2d only
3D merge- 3D only
Pmerge- To particles -----------> does this work when you have many particles and you want to make it into one or does 3D merge work with particles as well???
merge- 2d only
3D merge- 3D only
Pmerge- To particles -----------> does this work when you have many particles and you want to make it into one or does 3D merge work with particles as well???