I'm seeing \dt used for "show tables", \l used for "show databases". Why not standart SQL syntax words? Why specified PostgreSQL commands? I can't figure out that.
As others pointed out, "show databases" and "show tables" are not from any standard.
Since you seem to come from MySQL, here is a list terms translated to PostgreSQL:
Andreas 'ads' Scherbaum German PostgreSQL User Group European PostgreSQL User Group - Board of Directors Volunteer Regional Contact, Germany - PostgreSQL Project