Difference Between Update and Synchronize

Global doesn't "exchange" entire databases.
It just "synchronize" them.
Only "differences" are sent across from Hub to Sats and viceversa.
So a relatively small amount of data is passed on the network (it depends also on other factors like band width, synchronization frequency, etc etc )

Original Credit :- Corrado Galluzzi 

Update mean which will be overwritten the complete information ??

Sync mean  only difference of the changes will be done ??

when we should perform Update and Sync??Corrado Galluzzi 

I think there is a misunderstanding....

If you are talking about Global there is no difference.

I mean...you create an "update event" in order to synchronise one or more databases.

This comes from Administering Global Projects Training Manual:

"Update events are used to synchronise AVEVA Databases and these events are two-way. Update event can be created at the Hub or the Satellite and is normally created at the owning location."

Hope this helps

Thank you Corrado Galluzzi  for detailed explanation