Understanding Relations

As usual, the PostgreSQL documentation provides us with some information here in its section entitled the FROM Clause:

Note: A table reference can be a table name (possibly schema-qualified) or a derived table such as a subquery, a join construct, or complex combinations of these. If more than one table reference is listed in the from clause, the tables are cross-joined (that is, the Cartesian product of their rows is formed; see below). The result of the from list is an intermediate virtual table that can then be subject to transformations by the where, group by, and having clauses and is finally the result of the overall table expression.