PostgreSQL is implemented using a
    simple “process per user” client/server model.  In this model
    there is one client process connected to
    exactly one server process.  As we do not
    know ahead of time how many connections will be made, we have to
    use a master process that spawns a new
    server process every time a connection is requested. This master
    process is called postgres and listens at a
    specified TCP/IP port for incoming connections. Whenever a request
    for a connection is detected the postgres
    process spawns a new server process. The server tasks
    communicate with each other using semaphores and
    shared memory to ensure data integrity
    throughout concurrent data access.
   
The client process can be any program that understands the PostgreSQL protocol described in Chapter 53. Many clients are based on the C-language library libpq, but several independent implementations of the protocol exist, such as the Java JDBC driver.
Once a connection is established the client process can send a query to the backend (server). The query is transmitted using plain text, i.e., there is no parsing done in the frontend (client). The server parses the query, creates an execution plan, executes the plan and returns the retrieved rows to the client by transmitting them over the established connection.