Postgres table column name restrictions?

I did this in psql:

CREATE TABLE IF NOT EXISTS apiss (skey TEXT, time INTEGER, user TEXT, ip TEXT);

I get

ERROR:  syntax error at or near "user" LINE 1: ...BLE IF NOT EXISTS apiss (skey TEXT, time INTEGER, user TEXT,...

I do:

CREATE TABLE IF NOT EXISTS apiss (skey TEXT, time INTEGER, userd TEXT, ip TEXT);

It works.
Note the userd instead of user.

Are there some restrictions on the column names that a table can have? (postgresql v9.1.3)