increment
generates identifiers of type long, short or int that are unique only when no other process is inserting data into the same table. Do not use in a cluster.

identity
supports identity columns in DB2, MySQL, MS SQL Server, Sybase and HypersonicSQL. The returned identifier is of type long, short or int.

sequence
uses a sequence in DB2, PostgreSQL, Oracle, SAP DB, McKoi or a generator in Interbase. The returned identifier is of type long, short or int

hilo
uses a hi/lo algorithm to efficiently generate identifiers of type long, short or int, given a table and column (by default hibernate_unique_key and next_hi respectively) as a source of hi values. The hi/lo algorithm generates identifiers that are unique only for a particular database. Do not use this generator with connections enlisted with JTA or with a user-supplied connection.

seqhilo
uses a hi/lo algorithm to efficiently generate identifiers of type long, short or int, given a named database sequence.

uuid.hex
uses a 128-bit UUID algorithm to generate identifiers of type string, unique within a network (the IP address is used). The UUID is encoded as a string of hexadecimal digits of length 32.

uuid.string
uses the same UUID algorithm. The UUID is encoded a string of length 16 consisting of (any) ASCII characters. Do not use with PostgreSQL.

native
picks identity, sequence or hilo depending upon the capabilities of the underlying database.

assigned
lets the application to assign an identifier to the object before save() is called.

foreign
uses the identifier of another associated object. Usually used in conjunction with a <one-to-one> primary key association.