Q: iMovie Folders after upgrading to 10.0.1

I just upgraded to iMovie 10.0.1.  I clicked on the File Menu, the option for upgrading files and folders (i think thats what its called).  It starts converting but fails after a particular one.  So I decided to move out my projects from the iMovie Projects folder except a small one, ran the upgrade again and it converted it perfectly to iMovie 10.  So I went to look in finder and i have a folder structure Im not entirely clear about:

Inside Movies folder I have:

iMovie Library ~ 21 GB

iMovie Events ~ 18 GB

myoldiMovieProjects ~ 245MB (this is where i moved all my old iMovieProjects except one which was successfully converted

iMovieProjects ~162KB which only contains my one successfully converted iMovieProject and a file called UpdatedToiMovie10

My question is, since Im on a file cleaning binge...whats up with iMovie Events?  Whats the difference between that and iMovieLibrary?