The best description of pgAdmin III can be found on the project's website:
"pgAdmin III is the most popular and feature rich Open Source administration and development platform for PostgreSQL, the most advanced Open Source database in the world. The application may be used on Linux, FreeBSD, Solaris, Mac OSX and Windows platforms to manage PostgreSQL 7.3 and above running on any platform, as well as commercial versions of PostgreSQL such as Pervasive Postgres, EnterpriseDB, Mammoth Replicator and SRA PowerGres."
"pgAdmin III is designed to answer the needs of all users, from writing simple SQL queries to developing complex databases. The graphical interface supports all PostgreSQL features and makes administration easy. The application also includes a syntax highlighting SQL editor, a server-side code editor, an SQL/batch/shell job scheduling agent, support for the Slony-I replication engine and much more. Server connection may be made using TCP/IP or Unix Domain Sockets (on *nix platforms), and may be SSL encrypted for security. No additional drivers are required to communicate with the database server."
"pgAdmin III is developed by a community of PostgreSQL experts around the world and is available in more than a dozen languages. It is Free Software released under the Artistic License."
Those who use pgAdmin III find it to be invaluable for performing database maintenance, along with simply examining database structures and data.