A column can be assigned a default value. When a new row is created and no values are specified for some of the columns, those columns will be filled with their respective default values. A data manipulation command can also request explicitly that a column be set to its default value, without having to know what that value is. (Details about data manipulation commands are in Chapter 6.)
In a table definition, default values are listed after the column data type. For example:
CREATE TABLE products ( product_no integer, name text, price numeric DEFAULT 9.99 );
The default value may be an expression, which will be evaluated whenever the default value is inserted (not when the table is created). A common example is that a timestamp column may have a default of now(), so that it gets set to the time of row insertion. Another common example is generating a "serial number" for each row. In PostgreSQL this is typically done by something like
CREATE TABLE products ( product_no integer DEFAULT nextval('products_product_no_seq'), ... );
where the nextval() function supplies successive values from a sequence object (see Section 9.12). This arrangement is sufficiently common that there's a special shorthand for it:
CREATE TABLE products ( product_no SERIAL, ... );
The SERIAL shorthand is discussed further in Section 8.1.4.