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 using WITHOUT OIDS, in which case this column is not present). This column is of type oid (same name as the column); see Section 8.11 for more information about the type.
The OID of the table containing this row. This column 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 row version. (A row version is an individual state of a row; each update of a row creates a new row version 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 row version. It is possible for this column to be nonzero in a visible row version: 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 row version within its table. Note that although the ctid can be used to locate the row version 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.
OIDs are 32-bit quantities and are assigned from a single cluster-wide counter. In a large or long-lived database, it is possible for the counter to wrap around. Hence, it is bad practice to assume that OIDs are unique, unless you take steps to ensure that they are unique. Recommended practice when using OIDs for row identification is to create a unique constraint on the OID column of each table for which the OID will be used. Never assume that OIDs are unique across tables; use the combination of tableoid and row OID if you need a database-wide identifier. (Future releases of PostgreSQL are likely to use a separate OID counter for each table, so that tableoid must be included to arrive at a globally unique identifier.)
Transaction identifiers are also 32-bit quantities. In a long-lived database it is possible for transaction IDs to wrap around. This is not a fatal problem given appropriate maintenance procedures; see Chapter 21 for details. It is unwise, however, to depend on the uniqueness of transaction IDs over the long term (more than one billion transactions).
Command identifiers are also 32-bit quantities. This creates a hard limit of 232 (4 billion) SQL commands within a single transaction. In practice this limit is not a problem --- note that the limit is on number of SQL commands, not number of rows processed.