Every table has several system columns that are implicitly defined by the system. Therefore, these names cannot be used as names of user-defined columns. (Note that these restrictions are separate from whether the name is a key word or not; quoting a name will not allow you to escape these restrictions.) You do not really need to be concerned about these columns, just know they exist.
The object identifier (object ID) of a row. This is a serial number that is automatically added by PostgreSQL to all table rows (unless the table was created WITHOUT OIDS, in which case this column is not present). This column is of type oid (same name as the column); see Section 5.10 for more information about the type.
The OID of the table containing this row. This attribute is particularly handy for queries that select from inheritance hierarchies, since without it, it's difficult to tell which individual table a row came from. The tableoid can be joined against the oid column of pg_class to obtain the table name.
The identity (transaction ID) of the inserting transaction for this tuple. (Note: In this context, a tuple is an individual state of a row; each update of a row creates a new tuple for the same logical row.)
The command identifier (starting at zero) within the inserting transaction.
The identity (transaction ID) of the deleting transaction, or zero for an undeleted tuple. It is possible for this field to be nonzero in a visible tuple: That usually indicates that the deleting transaction hasn't committed yet, or that an attempted deletion was rolled back.
The command identifier within the deleting transaction, or zero.
The physical location of the tuple within its table. Note that although the ctid can be used to locate the tuple very quickly, a row's ctid will change each time it is updated or moved by VACUUM FULL. Therefore ctid is useless as a long-term row identifier. The OID, or even better a user-defined serial number, should be used to identify logical rows.