The backup dialogue presents a somewhat simplified interface to the PostgreSQL pg_dump tool. You can backup a single table, a schema or a complete database, dependent on the object you select when starting the backup tool.
pg_dump does not support all options for all backup file formats. Particularly, to backup blobs the PLAIN format can not be used. Also, a PLAIN file can not be interpreted, and can not be restored using pgAdmin. The PLAIN format will create an SQL script that can be executed using the psql tool. For standard backup and restore purposes, the COMPRESS and TAR options are recommended.
In order to use backup, the pg_dump tool must be accessible by pgAdmin. This can be accomplished by having it locatable using the path, or by copying it into the same directory where the pgadmin3 executable resides.