Backup & Recovery - PostgreSQL 9 Administration - page 267

You can also do this through pgAdmin3 as shown in the following screenshot:
How it works...
pg_dump produces a single output file. The output file can use the split(1) command to
separate the file into multiple pieces, if required.
pg_dump into custom format is lightly compressed by default. Compression can be removed
or made more aggressive.
pg_dump runs by executing SQL statements against the database to unload data. When
PostgreSQL runs an SQL statement we take a "snapshot" of currently running transactions,
which freezes our viewpoint of the database. We can't (yet) share that snapshot across
multiple sessions, so we cannot run an exactly consistent pg_dump in parallel in one
database, nor across many databases.
The time of the snapshot is the only time we can recover to—we can't recover to a time either
before or after that time. Note that the snapshot time is the start of the backup, not the end.