Pgpool-II is a proxy server sitting between clients and PostgreSQL. Pgpool-II understands the wire level protocol used by PostgreSQL called "frontend and backend protocol". For more details of the protocol, see the PostgreSQL manual. No modified PostgreSQL is required to use Pgpool-II (more precisely, you will need a few extensions to use full functions of Pgpool-II). So Pgpool-II can cope with variety of PostgreSQL versions. In theory, even the earliest version of PostgreSQL can be used with Pgpool-II. Same thing can be said to client side. As long as it follows the protocol, Pgpool-II happily accept connections from it, no matter what kind of languages or drivers it uses.
Pgpool-II consists of multiple process. There is a main process, which is the parent process of all other process. It is responsible for forking child process each of which accepts connections from clients. There are some worker process those are forked from the main process as well, which is responsible for detecting streaming replication delay. There is also a special process called "pcp process", which is solely used for management of Pgpool-II itself. Pgpool-II has a built-in high availability function called "watchdog". Watchdog consists of some process. For more details of watchdog, see Chapter 4.