Where is information about user-defined types stored?
Are there some tables which contain information about the fields of a user-defined composite type, their names, etc.?
The catalog pg_type stores information about data types. Base types and enum types (scalar types) are created with CREATE TYPE, and domains with CREATE DOMAIN.
More information about pg_type plz visit http://www.postgresql.org/docs/9.0/static/catalog-pg-type.html