When you create a table and you realize that you made a mistake, then you can drop the table and create it again. But this is not a convenient option if the table is already filled with data, or if the table is referenced by other database objects (for instance a foreign key constraint). Therefore PostgreSQL provides a family of commands to make modifications on existing tables.
You can
Add columns,
Add constraints,
Remove constraints,
Change default values,
Rename a column,
Rename the table.
In the current implementation you cannot
Remove a column,
Change the data type of a column.
These may be possible in a future release.