In Postgres, what is the relationship between a SEQUENCE and a SERIAL?

Question

Postgres has the ability to define SERIAL datatypes and SEQUENCE but they seem to overlap in function. What are the main differences ?

Answer 1

serial is purely a shorthand way to create an integer column with an associated sequence for its default values.

The documentation for serial linked from the question even says as much:

The data types smallserial, serial and bigserial are not true types, but merely a notational convenience for creating unique identifier columns (similar to the AUTO_INCREMENT property supported by some other databases). In the current implementation, specifying:

CREATE TABLE tablename (
    colname SERIAL
);

is equivalent to specifying:

CREATE SEQUENCE tablename_colname_seq;
CREATE TABLE tablename (
    colname integer NOT NULL DEFAULT nextval('tablename_colname_seq')
);
ALTER SEQUENCE tablename_colname_seq OWNED BY tablename.colname;