Trigger Behavior in Transactions

Greetings,

Where in the documentation can I find information on the low-level
behavior of triggers that fire as a result of insert/update statements
inside of transactions?

Do the triggers fire at the point the transaction is COMMITted?  Or as
soon as the statements are encountered?  (If this is the case, how are
rollbacks done - does the trigger also fire for the rollback?)

I am creating a mechanism for updating PG databases remotely (from another
PG database) for a magazine article I'm writing, and I need to figure out
how to engineer the trigger mechamisms so they work properly.

Thanks,
-Josh