What's the difference between pgpool II replication and postgresql replication?
The built-in replication, provided by PostgreSQL itself, includes streaming replication, warm standby, and hot standby. These options are based on shipping Write-Ahead Logs (WAL) to all the standby servers. Write statements (e.g., INSERT, UPDATE) will go to the master, and the master will send logs (WALs) to the standby servers (or other masters, in the case of master-master replication).
pgpool, on the other hand, is a type of statement-based replication middleware (like a database proxy). All the statements actually go to pgpool, and pgpool forwards everything to all the servers to be replicated.
One big disadvantage with pgpool is that you have a single point of failure; if the server running pgpool crashes, your whole cluster fails.
The PostgreSQL documentation has some basic info on the various types of replication that are possible: https://www.postgresql.org/docs/current/different-replication-solutions.html