I want to know the differences between a global and a local index.

I'm working with partitioned tables about 10 millons rows and 40 partitions.
I know that when your table is partitioned and your index non-partitioned is possible that
some database operations make your index unusable and you have tu rebuid it, for example
when yo truncate a partition your global index results unusable, is there any other operation
that make the global index unusable??

I think that the advantage of a global index is that takes less space than a local and is easier to rebuild,
and the advantage of a local index is that is more effective resolving a query isn't it???

Any advice and help about local vs global index in partitioned tables will be greatly apreciatted.

Thanks in advance