Both the pgsimple and pgsnapshot schemas are PostgreSQL schemas utilising postgis extensions that are capable of storing snapshots of OSM data.
No history is maintained.
They can be populated by osmosis and kept up to date with osmosis daily, hourly and minute changesets.

The pgsimple and pgsnapshot schemas are forked versions of the same schema.
Up to version 5 of the schema they were known as the "simple" schema.
The pgsimple schema is the "simple" schema and is unchanged.
Since version 6, the pgsnapshot schema has deviated to include all tag information in hstore "tags" columns inside the parent entity tables.

The purpose of the pgsimple schema is to provide a simplistic and generic schema for storing OSM data in a PostGIS format.
The purpose of the pgsnapshot schema is similar but maximises performance through CLUSTERed indexes and embedded tag data.  It imposes additional programming complexity.

The following scripts are available for both schemas:
pgxxx_schema_0.x.sql - The schema creation script.
pgxxx_schema_0.x_bbox.sql - A script for adding way bbox column support.
pgxxx_schema_0.x_linestring.sql - A script for adding way linestring column support.
pgxxx_schema_0.x_action.sql - A script for adding an action table which is populated during changeset processing to allow derivative tables to be kept up to date.
pgxxx_load_0.x.sql - A script for importing PostgreSQL "COPY" files as produced by the osmosis --write-pgxxx-dump tasks.