The order of events is:
create - Triggered when the sortable is created
start - Triggered when sorting starts
activate - Triggered for every connected list on drag start
over - Triggered when a sortable item is moved into a sortable list
sort - Triggered during sorting
change - Triggered during sorting, but only when DOM position has changed
beforeStop - Triggered before sorting stops, placeholder/helper
update - Triggered when stopped sorting and DOM position has changed
deactivate - Triggered before sorting stopped, propagated to all possible connected lists
out - Triggered when a sortable item is moved away from a sortable list
stop - Triggered when sorting has stopped
There are other events which can occur too:
receive/remove - Triggered when items are moved between lists
No idea why this isn't clearer on the jQueryUI site.
Note that, depending upon the event triggered, the ui arguments differs. It's worth looking at the API documentation to be clear what this is and to debug it.
http://api.jqueryui.com/sortable/