This flow-chart shows the processes that FileMaker goes through when opening a file. It includes the details on from the user opening a file through to the point where FileMaker gives the user control of the UI again; this includes entering a user name, authentication, privilege sets, and script triggers. (But it mostly concentrates on the authentication process.)
EDIT: I have added some of the server-to-client downloading of items as mentioned by Chocohalic. Made visual changes that will (hopefully) make it a bit more clear.