Oracle

Get all tables including the system ones:
SELECT table_name FROM all_tables;

That will give you only the tables owned by the user that you are connecting as:
SELECT table_name FROM user_tables;

Query table columns:
SELECT * FROM all_tab_cols WHERE table_name = 'whatever table name you are looking for'

Looking for comments on the tables:
SELECT * FROM all_tab_comments WHERE table_name = 'whatever table name you are looking for'

SQL-Server

That will give you all tables:
SELECT * FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_TYPE = 'BASE TABLE'